Because they're stupid, or at the very least have been brought up in a restrictive culture or family and never matured enough mentally to question it.

Stupid might seem a harsh word, but you know, there really isn't a single legitimate reason to disbelieve in homosexuality. Biologically, it's been proven that same-sex pheromones act differently on a gay man or woman, giving them the same reaction that it gives a straight person of the opposite sex. In zoology, hundreds of animals have shown homosexual behavior, everything from bonobos to fireflies to dolphins, and back again. In archaeology, there is no period of well-recorded history from the beginning of recorded history without some mention of homosexuality. A lot of it's debated, usually by staunchly religious men or women who practice revisionism, but quite a lot no one in their right mind would debate, since, after all, we have graphic depictions of men and men or women and women having sex for about as long as we've had art. In terms of sociology, I could start listing cultures for whom homosexuality was natural, accepted, even revered, including cultures that are alive and well today.

Theology is one thing, but even there, most of the people (particularly Christians) who "disbelieve" in homosexuality do so because they've never taken the time to question the preachings of homophobic priests, and actually look at the messages of the bible, or learn one iota of the history of their own religion. The bible has three arguable passages on the subject: Leviticus, which is so well-refuted I don't feel like I need to get into it (he was just writing an incredibly outdated code of behavior, behavior that was recorded in late antiquity, thousands of years ago, and why people think it makes sense to follow it today I'll never understand - religious people ignore most of it anyway), Sodom & Gomorrah (the point of which was more about promiscuity and rape than homosexuality, is part of the old testament anyway, you know, the section that advocates the stoning of unruly children, and which always makes me laugh because bible literalists actually think that happened [and once upon a time in the historical town of crazy bisexual rapists...]), and Paul's letters, which, honestly, guys, Paul NEVER MET JESUS, and was a missionary, not a direct mouthpiece of God. The idea of homosexuality being "against your religion" is thus condensed stupidity, coming from bigotry that was cooked up in the middle ages and then became a part of the religion practically de facto. The almost completely ignored stipends against murder are written in your ten commandments. Homosexuality? Practically a footnote.

And then of course we could get into what I call the "women and veils argument," and talk about how just because something is forbidden in YOUR sect of YOUR religion, common sense says not everyone belongs to your particular sect of your particular religion, and expecting people to follow it, particularly making laws in a government setting to force people to follow it without logical, defensible proof that the law is beneficial to society as a whole, is ethically indefensible. if you think it's wrong that a woman in Iran found without a veil can be sent to jail, no matter what her views on the veil or religious beliefs, then following the same logic you should think it's wrong for the government to try to regulate the life and marriage of and give inferior rights to a gay couple as opposed to a straight one, because the law-makers happen to follow in their private life a certain brand of Christianity.

So, to come to some sort of conclusion here, the reason people don't believe in homosexuality is because they're too entrenched in bigotry or prejudice to realize how stupid they're being. If they bothered to learn, for example, how one defines a mental disorder (which some fools think homosexuality is), they'd know a mental illness CANNOT be a mental illness unless it significantly negatively affects one's life (which is why there's a difference between being a neat freak and being OCD). Other than the SOCIETAL backlashes, which aren't universal and aren't intrinsent to the gay man or woman, but to the person discriminating against them, nobody has been able to make a legitimate argument that homosexuality causes any kind of significant harmful effects in the gay person's life. In fact, the majority of gay people live happy, healthy, productive lives at almost the exact same rate as heterosexual people, particularly in places where there's less prejudice. Being gay hasn't shown any major deformality in the structure of a gay person's brain - hormones are slightly different, but a pregnant or menstruating woman has more significant negative effects because of hormones than your average gay person. It wasn't "an empty political move" when the American Psychiatric Association took homosexuality off of the list of diagnosable disorders, and all major psychiatric organizations followed suit - it was because being gay is about as much a mental disorder as being straight is.

Ironically, the so-called "gay treatments" men, women, and children are given actually have a significant and sordid history of increasing risk of suicide, self-harm, major depression (mostly situational), and feelings of alienation and self-loathing in otherwise mentally sound patients. It's like giving a healthy person chemo: the methods of psychiatric treatment they're using are occassionally parodies of legitimate practices, just as chemo actually works on people who have cancer, but on a person who has no or unrelated psychiatric issues, it's just a way of destroying self-esteem, just like chemo is poison to anyone who doesn't have cancer.