It isn't; all sins are equally sinful. As one of the previous answers said, the portions of the bible against homosexuality also involve restricting marriages to virgins only and so forth -- eating shellfish is also something the OT forbids, but no one is outlawing shrimp or oysters. If anything, it is not a sin any longer because the New Testament was supposed to replace the Old -- wearing mixed fabrics is no longer a sin, is it? Homophobia is not a "made up word" by any means and quite frankly the answers you've received pretty much confirm it's existence, which is a real shame.
The real issue here is that people feel the need to be above someone else; they can't stand that they aren't better than someone, and violate their precious bible's supposedly most important teaching -- love everyone. Loving everyone does not mean persecuting a group of people for being different any more than being not racism means hanging black people and refusing to let them drink from a 'white' water fountain.
Please; if you're asking this question, it's a good thing because it shows that you're thinking critically about an important civil rights issue plaguing us today. Don't stop doing that. The world can never improve if no one is willing to take a good look at what's going on and ask "why are we being horrible to _____?" It's important to, if you're of voting age, look to the laws currently being considered; especially those on topics like this. Legal marriage gives the spouse a lot of important benefits, like medical stuff and being able to take care of their loved one. If you're in America, this shouldn't even be a question because we live in a secular country, founded by Deists to be secular. Christianity should not be able to dictate the law, and we are not a Christian country. Homosexuality is NOT a question of wrong gender but a sexual preference, and it is persecution and prevention simply to deny them the equal right to marry the person they love. Marriage is not sacred, just look at the number of divorces and so forth. There is literally no secular reason not to allow them this.
My best advice, of course, is to read the bible itself and see what is written specifically. I don't have a bible on me or the time to go searching through an online one, but I can honestly tell you that very, very man 'Christians' haven't actually read what is written. It also outlaws masturbation, but how many people do you think would be furious if that became illegal?
edit; Atheists focus on Christians because they are one of the most vocal groups and tend to be one of the most thoughtless within their cultural context in countries like America that are supposed to be based on freedom. To be quite honest, most atheists that I have met know more about Christianity than self-proclaimed Christians do because they have bothered to do the research and think about it and decide for themselves. Muslims have nothing to do with the question, and bringing them or any other religion except MAYBE Judaism into the question is discriminatory and pathetic. :\