|My favorite desert was the Sahara|
As a child, I had already mastered confirmation bias. Confirmation bias is the tendency to seek out and accept evidence that confirms what we already believe or want to believe, and reject evidence that contradicts those beliefs. It's so powerful that when participants in controlled experiments are given fabricated evidence (unknown to them that it's fake) specifically designed to disprove their beliefs, they interpret it as actually supporting their beliefs. Bible interpretation is rife with confirmation bias. One example is the debate over same-sex marriage.
Many opponents of gay marriage point to commandments like Leviticus 18:22 & 20:13, which say a man should not "lie" with a man as one would with a woman, as proof that homosexuality is wrong and thus gay marriage should be illegal. Some mention the fact that God created humans male and female and suggest that God defines marriage as between one man and one woman.
Bible-believing proponents of gay marriage have found convenient ways around biblical commandments, such as pointing out commandments against wearing clothes of different fabrics and planting fields with different seeds (both in Leviticus 19) or about eating certain foods like shellfish. Others find support for gay marriage in the words of Jesus. Some even suggest the close relationship between David and Jonathan was more than just friendship.
All of these are examples of overfitting and confirmation bias, which often reinforce each other. People have strong opinions about homosexuality and gay marriage for cultural reasons. That was true before the Bible was written. Currently, the best predictor of one's opinion about gay marriage is age. According to recent polls, 70% of Americans aged 18-29 support it, compared to only 41% who are 65 and older. Even the majority of Republicans aged 18-29 support gay marriage, a higher percentage than Democrats aged 65+. At least half of young evangelical Christians also support same-sex marriage, similar to older non-religious people.
As a strong supporter of gay marriage, I'll inevitably be accused of confirmation bias here, which I can't deny. Nevertheless, I'll try my best to avoid it while answering some common questions.
What does the Bible say about gay marriage?
Nothing. Gay marriage wasn't an issue when/where the Bible was written, so it wasn't directly addressed.
What does the Bible say about homosexuality?
Specifically, nothing. The prohibitions in Leviticus are of a very specific act, one that should not be equated with homosexuality or even practicing homosexuality. Those prohibitions had a very practical purpose at the time (i.e., longevity in the new land), as stated in the same chapters. Whether they still apply is debatable, but it's a different debate. Other mentions of same-sex activity in the Bible involve rape or prostitution, which are similarly condemned for heterosexuals. Even if the Bible did teach that homosexuality is wrong, that has never been the standard for legislation. Idolatry and blasphemy, for example, are clearly wrong according to the Bible, but almost nobody sees the First Amendment as an attack on biblical morality.
What about fabrics, seeds, and shellfish?
In my opinion, this is a bad and unnecessary argument for same-sex marriage. There's a major difference between these and the commandments given in Leviticus 18 & 20. Leviticus 18 was directed at both Israelites and foreigners (Lev. 18:26), but the next chapter (the one that prohibits different fabrics and seeds) was directed only at Israelites (Lev. 19:1), as were the dietary laws. This is an important distinction to Jews, who generally believe that non-Jews are only required to follow the 7 Laws of Noah, which include sexual immorality (Lev. 18) because it also applied to "foreigners", but not the ceremonial and dietary laws or those in Lev. 19. A similar belief was evident among early Christians (see Acts 15).
Does the Bible define marriage exclusively as one man and one woman?
No. Biblical marriage often was between one man and multiple women. Among the many examples are Abraham, Jacob, David, and probably even Moses. Rather than condemning it, the Bible gives provisions for how to treat multiple wives (e.g., do not neglect your first wife after you get another one; Ex. 21:10). Ironically, some Bible-believers argue that gay marriage is a slippery slope that could lead to legalization of polygamy, perhaps forgetting that the Bible allowed it.
Gay marriage is a controversial political and cultural issue. As much as we'd like the Bible to settle all of our political and cultural debates, its authors had a different purpose. That doesn't mean the Bible is irrelevant or that everything is morally neutral unless there's a specific commandment against it. But we need to be very careful to allow the text to speak for itself rather than using it to confirm our own beliefs. The Bible doesn't give direct commandments about everything, but it does give one (also in Leviticus) that summarizes all of them: "love your neighbor has yourself." That one sometimes is forgotten by people on both sides of the gay marriage debate.