Read through the materials of just about any Christian organization that speaks out against gay relationships, and you'll almost certainly find reference to Leviticus 18:22. The wordings of our English translations of the verse appear very clear, and at face value lend weight to those groups' political crusade against gay rights. One can, after all, hardly speak positively about anything that God called an 'abomination.'
Of course, there are many other 'abominations' listed in the Old Testament, shellfish being the most famous example, so conservative theologians have been forced to refine their arguments to explain why some of Moses' commands are still applicable today and some are not. To facilitate this, a third category was added to the Law; in addition to the ceremonial laws and the purity laws, they argued, there are also universal moral laws.
Whereas the ceremonial and purity laws are easily distinguishable based on context, the primary criterion for determining whether a command falls into the universal moral law is whether it appears to be something that most Christians would agree ought to be considered wrong under any circumstances. Since such acts as murder, theft, adultery and incest are clearly wrong regardless of whether one is an observant Jew, then commands found against those activities must be exceptions to the freedom that the New Testament grants Christians from the Law.
The problem with this approach is that it requires one to pick individual verses out of the Pentateuch while in many cases ignoring other verses immediately before and after. Not in every case - one can make a reasonable case for placing the Ten Commandments in the category of universal law, and few Christians would dispute such a claim - but even here we have a command (observing the Sabbath) that carries little or no weight in many churches.
Of course, the Ten Commandments say nothing about homosexuality, unless one makes the questionable stretch of expanding the definition of 'adultery' far enough to include it. Such a stretch is necessary, we are told, because one wouldn't want to be forced to defend acts of incest and bestiality just because they weren't specifically mentioned in the Ten Commandments.
Conservatives do have one option for supplementing the seventh commandment ("you shall not commit adultery") without cherry-picking individual verses; Leviticus contains two chapters (18 and 20) filled with prohibitions that even many outside the faith would agree with: adultery, various forms of incest, bestiality, human sacrifice and, of course, (male) homosexual sex. Chapter 19 has to be excluded from this context, due to the hodgepodge of odd commands it contains, but one can always go back to cherry-picking to rescue its one relevant command ("Love your neighbor as yourself").
Unfortunately, we're still left with one odd prohibition in the mix: Lev. 18:19 and its echo in 20:18, which forbids a man from sleeping with a woman during her period. Some modern-day Christians probably aren't even aware of this command, and it's certainly not taken seriously by the majority of those that are. But contextually it cannot be separated from the rest of the commands in those chapters; in chapter 18 it falls between the commands against incest and adultery, and in chapter 20 in the middle of the incest prohibitions.
Granted, offenders don't receive the death penalty, but exile was only a marginally better fate in ancient times. Exile was also the punishment for several different forms of incest, so clearly this command isn't to be taken any less seriously just because the death penalty isn't called for. And granted, it isn't repeated in Deuteronomy (as most of the commands in Leviticus are), but neither is the prohibition against men lying with men unless one ties it to the command against male and female "holy ones" (Deut. 23:17). Since such a connection would place the Old Testament's only direct reference to homosexuality firmly within the context of pagan fertility rites, it's clear that we can't afford to put too much weight on what is and isn't repeated in Deuteronomy.
And the command against sex during menstruation is mentioned elsewhere in the Bible; Ezekiel does so twice (Ezek. 18:6 and 22:10), both times in conjunction with other moral issues (adultery, incest, idolatry and various economic injustices). No doubt the first instinct of most readers would be to dismiss this particular command as a relic from the purity laws, but the contexts it's mentioned in rule out that possibility.
Some might argue that having sex with a woman during her period must not be a big deal since it wasn't mentioned in the New Testament, but bestiality isn't mentioned there either, and it's hard to imagine bestiality advocates finding many supporters within the church. Furthermore, as we so frequently hear in the debate over homosexuality, just because an issue wasn't mentioned by Jesus (or, for that matter, Paul) doesn't mean it's unimportant. The authors of the New Testament may simply have considered it so obvious that didn't need to be repeated to their audience. In fact, they probably did; most premodern societies contained strong taboos against sleeping with a woman during her period.
Defenders of the sex-with-my-wife-whenever-we-want-it lifestyle would probably fall back on the biblical principle that "the marriage bed is undefiled" to defend their proclivities, but allowing generalized principles to override clearly stated biblical commands opens the door to a potentially endless flood of exceptions and qualifications. A law like this one may not make sense to us right now, but surely God would not issue a command without good reason. If we truly believe that the Bible contains explicit instructions for how we are to order our lives, then the believer's duty is to obey every one of God's commands even if we never understand why in this lifetime.
And some researchers have determined that sex during menstruation may have negative side effects. Some others may disagree, but again, God undoubtedly has a good reason for issuing a prohibition.
Furthermore, church tradition was unanimous on this issue all the way into the mid-19th century. Many of the church fathers and major theologians, including Jerome, Clement of Alexandria, Augustine, St. John Chrysostom and Thomas Aquinas condemned the practice, and none tried to defend it. It's only in modern times that some Christians have begun to go astray and abandon this universally-held position.
Or could it be that combing through Leviticus for relevant commands causes us to miss the point entirely? Perhaps, by creating arbitrary categories within the Law to satisfy our desire for a rules-based approach to faith, we run afoul of Paul's admonition in Galatians 3:10-12:
All who rely on observing the Law are under a curse, for it is written: "Cursed is everyone who does not continue to do everything written in the Book of the Law." Clearly no one is justified before God by the Law, because, "The righteous will live by faith." The Law is not based on faith; on the contrary, "The man who does these things will live by them."
As followers of Christ we are free from the Law - all of the Law - not because morality is unimportant but because morality is diminished when we reduce it to an arbitrary set of rules. True Christian morality flows outward from the heart; one can appear devout on the surface, consistently obeying every last command issued by their church, and secretly remain an enemy of God. One can even use the letter of the law to justify actions that blatantly violate its spirit.
And strict adherence to the "plain language" of the Bible has caused problems of its own. American and European defenders of the institution of slavery had the letter of the law on their side whenever the Bible was brought into the debate, while abolitionists were forced to argue from abstract principles, yet how many Christians today would defend the notion that slavery is ever acceptable?
That still leaves plenty of room for debating what does and doesn't fall within the bounds of Christian ethics, but if some of the commands in Leviticus are central to that discussion, then all of them are - shellfish, management of slaves, menstrual cycles and all. Jesus may not have abolished the Law, but he did fulfill its requirements and free us from them all the same.