The Old Testament (OT)
The first mention of homosexual acts in passages about destruction of the Sodom and Gomorrah (Genesis 19). Proponents of this crazy theory claims, that sin of men who wanted to have homosexual sex with angels posing as male visitors of the City was not sin of homosexuality, but sin of rape.However this idea is complete novelty. Nobody in OT Israel would even consider homosexual relations to be moral. Why would the Scriptures then emphasized that they desired specifically the men and not the women (Lot's daughters). If consensual homosexual sex was accepted, this emphasize on homosexual nature of rape would be completely redundant. In other words this kind of interpretation does not fit presentation of the narrative.
The next and the most explicit condemnation of homosexual sex in OT is in the Mosaic Law.
Leviticus 20, 13 The man who has intercourse with a man in the same way as with a woman: they have done a hateful thing together; they will be put to death; their blood will be on their own heads. (The Catholic Bible on-line)
One of the arguments it that this verse speaks only about homosexual pagan rites. The second claim is that all command lay down in the Mosaic Law are no longer valid and to support this claim they pull out verses which on first look like complete nonsense nobody adheres to for a centuries.
This is a wishful-thinking if not outright lie. This chapter reflects the Decalogue (the ten commandments). First verses are condemning the practice of child sacrifice (killing) for Moloch (a god). The key verse to understand the purpose of Mosaic law follows.
Leviticus 20
7 Sanctify yourselves and be holy, for I am Yahweh your God.
8 You will keep my laws and put them into practice, for it is I, Yahweh, who make you holy.
What follows is list of straightforward rules on relations in family, sexual morality included.
Leviticus 20
9 Hence: Anyone who curses father or mother will be put to death. Having cursed father or mother, the blood will be on that person's own head.
10 The man who commits adultery with his neighbour's wife will be put to death, he and the woman.
11 The man who has intercourse with his father's wife has infringed his father's sexual prerogative. Both of them will be put to death; their blood will be on their own heads.
12 The man who has intercourse with his daughter-in-law: both of them will be put to death; they have violated nature, their blood will be on their own heads.
13 The man who has intercourse with a man in the same way as with a woman: they have done a hateful thing together; they will be put to death; their blood will be on their own heads.
14 The man who marries a woman and her mother: this is incest. They will be burnt alive, he and they; you will not tolerate incest.
15 The man who has intercourse with an animal will be put to death; you will kill the animal too.
16 The woman who approaches any animal to have intercourse with it: you will kill the woman and the animal. They will be put to death; their blood will be on their own heads.
17 The man who marries his father's or his mother's daughter: if they have intercourse together, this is an outrage. They will be executed in public, for the man has had intercourse with his sister; he will bear the consequences of his guilt.
18 The man who has intercourse with a woman during her monthly periods and exposes her nakedness: he has laid bare the source of her blood, and she has exposed the source of her blood, and both of them will be outlawed from their people.
19 You will not have intercourse with your mother's sister or your father's sister. Whoever does so, has had intercourse with a close relation; they will bear the consequences of their guilt.
20 The man who has intercourse with the wife of his paternal uncle has infringed his uncle's sexual prerogative; they will bear the consequences of their guilt and die childless.
21 The man who marries his brother's wife: this is pollution; he has infringed his brother's sexual prerogative; they will die childless.
In this context it is completely bogus to claim this single commandment is not condemnation of homosexual sexual activities and other morally abhorrent behavior in the eyes of the God. In that regard proponents of this theory also must accept zoophilia/beastiality and incest. Notice that verses about seashells, sowing two seeds together, etc are not here in this Chapter. One more very important thing. Notice also that the first rule is about cursing your parents and every following verse in this part is not about pagan practices, but about family. Using simple logic you can clearly come into conclusion that adulterous, incestuous or homosexual relationships shouldn't be recognized as acceptable form of family. The following verses explain more the position God in relation to the Mosaic Law.
22 You will keep all my laws, all my decisions, and put them into practice, so that the country where I am taking you to live will not vomit you out.
23 You will not follow the laws of the nations whom I am driving out before you; they practised all these things, which is why I detested them.
The verses condemning seashells, two kinds of seeds, etc. Condemn particular pagan practices and customs and are literally on different page. The same can be said about laws concerning OT rite.
The New Testament (NT)
But, wait! That is in the Old testament. It is no longer valid.No, that is simplistic understanding or overstatement. The Old Covenant was binding people of Israel as holy chosen people of God. The sacrifices of the animals couldn't bring complete salvation (restoration) of mankind. That is why Saint Paul says the works (rites) of the (Mosaic) Law can not save you. However the morality of acts is still same before God. God finds those actions detestable in other words sinful. That is why Jesus Christ can say:
Mathew 5
17 Do not imagine that I have come to abolish the Law or the Prophets. I have come not to abolish but to complete them.
18 In truth I tell you, till heaven and earth disappear, not one dot, not one little stroke, is to disappear from the Law until all its purpose is achieved.
19 Therefore, anyone who infringes even one of the least of these commandments and teaches others to do the same will be considered the least in the kingdom of Heaven; but the person who keeps them and teaches them will be considered great in the kingdom of Heaven.
It is our Lord Jesus Christ who deals a deadly blow to the idea of homosexual relationship as viable alternative by revealing intention of the Father.
Mathew 19
3 Some Pharisees approached him, and to put him to the test they said, 'Is it against the Law for a man to divorce his wife on any pretext whatever?'
4 He answered, 'Have you not read that the Creator from the beginning made them male and female
5 and that he said: This is why a man leaves his father and mother and becomes attached to his wife, and the two become one flesh?
6 They are no longer two, therefore, but one flesh. So then, what God has united, human beings must not divide.'
7 They said to him, 'Then why did Moses command that a writ of dismissal should be given in cases of divorce?'
8 He said to them, 'It was because you were so hard-hearted, that Moses allowed you to divorce your wives, but it was not like this from the beginning.
9 Now I say this to you: anyone who divorces his wife -- I am not speaking of an illicit marriage -- and marries another, is guilty of adultery.'
10 The disciples said to him, 'If that is how things are between husband and wife, it is advisable not to marry.'
11 But he replied, 'It is not everyone who can accept what I have said, but only those to whom it is granted.
12 There are eunuchs born so from their mother's womb, there are eunuchs made so by human agency and there are eunuchs who have made themselves so for the sake of the kingdom of Heaven. Let anyone accept this who can.'
This intention of marriage and in effect human sexuality clearly mentions solely man and woman joined together as one in procreational manner. Our Lord had no problem to speak his mind. Therefore any desire of homosexual relationships by God would be mentioned. Moreover it would be clear contradiction to the intent of upholding and fulfilling the Law.
side note: That is also one of reasons why the idea of "homosexual marriage" is repulsive to the Church. It is a revolt against God, against his design. It has handwriting of Satan all over. It is him who turns everything upside-down in God's order.
It is quite obvious that after such statements Jesus Christ never had to address issue of homosexuality. Especially in the Israel, which had Mosaic law for thousands of years It was no-brainer.
St. Paul
St. Paul confronted with immorality in pagan areas, where Christian faith was spreading addressed issue of homosexuality in his letters. The first is in letter to Romans.Romans 27
why their women have exchanged natural intercourse for unnatural practices; and the men, in a similar fashion, too, giving up normal relations with women, are consumed with passion for each other, men doing shameful things with men and receiving in themselves due reward for their perversion.
While most people know this short passages concerning homosexuality, let me put it in whole context, because it is much more interesting. The Saint Paul writes about Greeks, who were intellectually oriented society, the very same characterization is befitting for present society which is very steeped in rationalism. He does not only write about the spread of homosexuality among the Greeks, but also describes the state of society, which led to such thing.
Romans
19 For what can be known about God is perfectly plain to them, since God has made it plain to them:
20 ever since the creation of the world, the invisible existence of God and his everlasting power have been clearly seen by the mind's understanding of created things. And so these people have no excuse:
21 they knew God and yet they did not honour him as God or give thanks to him, but their arguments became futile and their uncomprehending minds were darkened.
22 While they claimed to be wise, in fact they were growing so stupid
23 that they exchanged the glory of the immortal God for an imitation, for the image of a mortal human being, or of birds, or animals, or crawling things.
24 That is why God abandoned them in their inmost cravings to filthy practices of dishonouring their own bodies-
25 because they exchanged God's truth for a lie and have worshipped and served the creature instead of the Creator, who is blessed for ever. Amen.
26 That is why God abandoned them to degrading passions:
27 why their women have exchanged natural intercourse for unnatural practices; and the men, in a similar fashion, too, giving up normal relations with women, are consumed with passion for each other, men doing shameful things with men and receiving in themselves due reward for their perversion.
28 In other words, since they would not consent to acknowledge God, God abandoned them to their unacceptable thoughts and indecent behaviour.
29 And so now they are steeped in all sorts of injustice, rottenness, greed and malice; full of envy, murder, wrangling, treachery and spite,
30 libellers, slanderers, enemies of God, rude, arrogant and boastful, enterprising in evil, rebellious to parents,
31 without brains, honour, love or pity.
32 They are well aware of God's ordinance: that those who behave like this deserve to die -- yet they not only do it, but even applaud others who do the same.
Nobody sane would claim that Saint Paul viewed homosexuality as an alternative to heterosexuality or in any positive way for that matter. Instead he speaks clearly that it is the punishment for abandoning the God (So called post-christian society anyone?) for worldly pleasures and pseudo-intellectual ideas. It is a symptom of large decay in society.
Next condemnation of homosexual sexual life is condemned in the first letter to Corinthians, Chapter 6.
1 Corinthians 6
9 Do you not realise that people who do evil will never inherit the kingdom of God? Make no mistake -- the sexually immoral, idolaters, adulterers, the self-indulgent, sodomites,
10 thieves, misers, drunkards, slanderers and swindlers, none of these will inherit the kingdom of God.
In the first letter to the Timothy there is nothing left to doubt about, that Saint Paul sees homosexuality as immoral and contrary to the teachings of Christianity.
1 Timothy 1
6 Some people have missed the way to these things and turned to empty speculation,
7 trying to be teachers of the Law; but they understand neither the words they use nor the matters about which they make such strong assertions.
8 We are well aware that the Law is good, but only provided it is used legitimately,
9 on the understanding that laws are not framed for people who are upright. On the contrary, they are for criminals and the insubordinate, for the irreligious and the wicked, for the sacrilegious and the godless; they are for people who kill their fathers or mothers and for murderers,
10 for the promiscuous, homosexuals, kidnappers, for liars and for perjurers -- and for everything else that is contrary to the sound teaching
11 that accords with the gospel of the glory of the blessed God, the gospel that was entrusted to me