One thing anti-homosexual advocates point to when trying to make their case is the Bible's condemnation of homosexuality. Often it gets raised as a discussion-ender--the Bible prohibits man laying with man, therefore as a Christian (or conservative Jew) you have to oppose it as well. QED.
Of course, this shouldn't be conceded, particularly when we're talking about a very long and complex bit of writing from thousands of years ago. (Consider also that the U.S. Constitution, which is a tiny fraction of the length of the Bible, written only a couple hundred years ago by people who are known to us, still is the subject of scholarly debate as to how it should be interpreted.) In a recent dustup with young Christians, columnist and gay activist Dan Savage basically argued that people should disregard the "bullshit" about gays in the Bible the same way they disregard other "bullshit" in the Bible, such as proscriptions on shellfish. The youngsters, offended, walked out of the event.
I'd actually take it a step further than Savage did though, and not even concede that the Bible--or a healthy belief in Christianity for that matter--is anti-homosexual. To accept this interpretation, consider what the actual purpose was for all the "rules" in the Old Testament. (This is where the condemnations of homosexual acts, as well as the shellfish ban, etc., came from. To the extent that the New Testament accepts any of these proscriptions, it is based on following the word of God in the Old Testament)
Everything God ordered his Chosen People to do in the Old Testament can be summed up in two general goals--maintain your loyalty to God, and do everything possible to prevent being wiped out or assimilated into any competing tribes. The "loyalty to Me" thing makes sense--without that, there's no point in being "Chosen" people, and the Israelites become no different from the Canaanites or Egyptians. Keeping from being wiped out meant procreating as much as possible--this is why if a husband dies then the brother should "lay with" the wife, and other weird hillbilly stuff that seems weird now but probably made sense for a small desert people surrounded by enemies and susceptible to dangerous primitive living conditions. This also meant avoiding "unclean" foods, such as shellfish or pork, that by 3000 BC standards could easily wipe out large groups of people with diseases. This also explained why God seemed to be okay with war against neighbors, which seems to violate the "Thou Shalt Not Kill" thing--no killing, unless it's necessary to protect the tribe.
With this context, one could understand why any form of non-procreative sex was banned--homosexuality, sodomy, etc.--every bit of sexual effort had to be channeled towards keeping the population up. Remember this was a time when plagues hit regularly, and health standards were low.
And based on that reasoning, it also makes sense that none of those condemnations from thousands of years ago actually mean that God would consider them applicable today--it's no longer necessary to maintain a fragile population, any more than it's unsafe to eat pork or lobster.
Granted, not everyone would agree with this interpretation, but by no means should anyone have to concede that one cannot be Christian (or any other Bible-based religion) without condemning homosexuality. Though as Savage suggests, if you're going to hang your justification for bashing gays on one of the Bible's many "rules", you should be ready to explain why you can ignore the rest of them.