Let us then begin with the affirmations, not prohibitions. Genesis 1-2 says the male was incomplete without the female: "It was not good for man to be alone." The woman was created to be a suitable companion for the man. Indeed, as the rabbis were to say later, the woman is seen as the crown of creation--that which not merely completes creation, but prompts the benediction that now finally all was tov m'od--very good.
Following this there is what has been called the creation order mandate-"Fill the earth and subdue it." This mandate could only be fulfilled by a complementary pair of creatures, male and female. Both were required for the propagation of the species, as well as for the assembling of enough humans to eventually subdue the earth.
Christian theology demands that we distinguish God's original design for creation from the effects of the fall: we can't assume that "whatever is, is right." Not all forms of sexual sharing, even between consenting adults, can be affirmed as either good or "normal", however "natural" certain desires may seem. When nature is out of joint, it can not serve as a clear barometer of what is good.
It has become fashionable to suggest that the famous story of Sodom, found in Genesis 19:4-8, and its close parallel in Judges 19:22-24, have no direct relevance to homosexuality. The sin of Sodom, it is said, was being inhospitable to strangers, even to the extreme of seeking to rape them.
This reading is possible, though the earliest interpreters of these texts, from the author of Jude to Josephus, did not seem to think so, and it is not certain that these texts can justify it. The verb yada "to know" is used to mean sexual sharing in the Sodom story, when Lot offers his daughters for them "to know" (i.e. have sexual intercourse with). It's probable that it means the same thing earlier, when the citizens of Sodom ask Lot to bring out his guests so they may "know" them. Lot accuses them of acting wickedly--an odd charge if all they meant is that they wanted to interrogate them to discover if they were spies.
The biblical echoes of the Sodom story, furthermore, all clearly refer to sexual sin. Jeremiah 49:18 criticizes Jerusalem for its sexual sin, and says it is like Sodom. Ezekiel 16:49 is sometimes cited to prove that the sin of Sodom was only inhospitality. But the next verse says the residents of Sodom were destroyed because they did "abominable things before me." Abominable, is the very term Lev. 18:22 and 20:13 uses to criticize homosexual intercourse.
Our next stop is the Gospels. Jesus never condemns homosexual behavior, but his words about marriage and singleness are very pertinent. In Matthew 19:1-12, Jesus offers two options in regard to sexual behavior--fidelity in marriage or celibacy in singleness. He suggests that divorce was not God's original intent for human beings, and that he was now reinforcing the original creational intent for men and women to be together in holy wedlock. He then suggests that anyone who cannot handle fidelity in marriage should either become (or remain) a eunuch, or stay celibate for the sake of the kingdom of God. The implication of absolutely no sexual sharing outside of marriage, whether of a heterosexual or homosexual sort, is very clear. Jesus is indeed asking for celibacy in singleness, whatever one's sexual predilections might be.
We turn now to Romans 1, and here indeed we have the most complete and clear discussion of the matter. Verse 27b could hardly be clearer: "Men commited shameless acts with men and received in their own persons the due penalty for their sin."
Paul used homosexual and lesbian sin as illustrations of the more general problem of the effects of the fall. For Paul, not unlike other early Jewish writers, homosexual behavior is perhaps the clearest example of how flouting sexual distinctions is ultimately a rejection of the Creator, who made such distinctions. In other words, it is not just immorality, it is idolatry.
Much of what Paul says could apply to any sort of sinner, for all sin is ultimately rebellion against God. But verses 26-27 illustrate how this rebellion plays itself out by taking the example of lesbian and homosexual sexual activity. At verse 26, Paul introduces the concept of nature into the discussion. The verse says literally "they have exchanged the natural use for that which is contrary to nature."