So basically, even among Christians the statement "homosexuality is a sin because the Bible says so" is up for A LOT of interpretation and debate.
I grew up an evangelical because "the Bible tells me so" about many issues. I've learned so much of it was misinterpreted, based on human bias, culture, etc etc.
I highly recommend this documentary: http://www.forthebibletellsmeso.org/indexe.htm . I'll admit when I watched it a couple years ago I still wasn't completely convinced (though I long took a libertarian stance of being fine with gay marriage as a government thing, because that was separate from my religious beliefs), but over two years this issue is resolved in my mind. Not to brag, but I have studied the Bible several times over, was an evangelical missionary, most of my church, family and friends still believe this way, etc...so I know all the arguments and scriptures about homosexuality being a sin, and I just don't buy it any more.
As some PP have pointed out some of these points:
#1: Soddom and Gomorrah's sin was neglecting the poor (watch out USA haha)
#2: the only sexual sin mentioned was gang rape, had nothing to do with the sexual behavior of consenting adults
#3: interestingly, female homosexuality is never mentioned in the Old Testament (my Mom likes to point out she thinks that's because God understood those poor lonely women who dozens of them were forced to marry the same man, of course He would let them find some fulfillment in each other haha)
#4: All Old Testament laws concerning homosexuality are ceremonial and smack dab next to not eating shellfish, wearing clothing of mixed fabric, etc. Why dismiss the latter two but keep the former?
#5: Jesus NEVER talked about homosexuality, so I doubt it was of great importance to Him. Homosexual acts were very prevalent in that society as well.
(Side note: this is my biggest reason for leaving evangelicalism (though I still claim Christianity, albeit a Univeralist/mystic/gnostic interpretation): the church claims to follow Christ but actually has become a Paul cult. Everything Jesus said is filtered through or expounded upon by Paul in our opinion. I dunno, if I personally am going to claim Christ, I am going straight to the source)
#6: New Testament passages dealing with homosexuality are very sketchy about the word interpreted, etc
#7: Paul, bless him, though inspired was still human...contradicted himself an awful lot and we shouldn't take the things he said as infallible. We can draw from him that he was ok with slavery, not ok with homosexuality and ok with oppressing women. Oh, also for the evangelicals who think the Bible is all about family values so work to legislate such, he was not all about family. He highly discouraged marriage all together. But he didn't sound exactly against Old Testament style polygamy. See, where he said elders should be "the husband of one wife" it was an issue of having enough time to devote to church ministry, he never actually said that it was a sin to be polygamous, even in his infamous list of sins including "homosexuality" that keep you out of the kingdom of God. On that note, remember culturally that "homosexuality" included man-boy forced relationships...I could go on and on.....
So my personal conclusion is that if you believe that the Bible is the 100% inerrant, infallible word of God AND that church history/tradition is authoritative then yes, you MIGHT be able to reason that homosexuality is a "sin". Even then, your case is weak. When you look at the modern science that proves it is NOT a choice, realize what a weak "case" the Bible has against it, if you believe in any form of a loving God...then well, you end up doing a 180 on the matter like I did =). God bless my homosexual sisters and brothers!