I like your call to civility. Your questions seem like they could lead to an interesting conversation in the midst of all this, so I'll bite.

What are the reasons you believe what you do? Interesting question. "What I believe" is a rather broad category, and it would take a long time to answer all of that. Let's see what the other answers provoke and come back to this one. --Well, I think I've usually included that in my answers.

What is the evidence that homosexuality is an in-born quality as opposed to a choice? How strong is it? There is my personal experience that I started being attracted to humans of one sex without consciously deciding to. I gather that this is a common experience for people of all sexual orientations, and I haven't heard of exceptions. There is also the logical consideration that if anyone was facing serious negative consequences for being gay and negligible benefits, and he could easily turn straight, then he would probably do it; I don't have the literature in my hands, but my impression is that if you look, say, a few decades in the past, you can find a fair number of gay people who (a) faced physical abuse and ostracism, (b) were very distressed by all this, (c) were not getting emotional support from the gay community because it didn't really exist yet, (d) could not plausibly be argued to have martyr complexes, and yet didn't "just stop being gay". Seems like strong evidence, though the underlying mechanism remains obscure.

That said, though, whether it's a choice is really moot in my opinion, and in fact something of a distraction. Orson Scott Card, a believing Mormon, wrote an entire essay where he starts from the premise that homosexual behavior is immoral, because his religion says so, and criticizes those who protest that the desire for it is innate: just because people have innate urges to do many other kinds of immoral behavior (including violence) doesn't make them moral. A fine argument if you agree with the premise, which his critics obviously do not. Since the entire point of contention is a matter of faith for him, it seems there is little to discuss with him, and the resulting debate is just awkward and embarrassing for all involved.

People have exclusive rights to their own bodies; they can form whatever voluntary sexual, emotional, financial, or other kinds of relationships, as far as I'm concerned; I would oppose anyone who would try to intervene by force, and would be wary of those who try to use shaming and other forms of social power to discourage it. Why people want what they want doesn't enter into this.

What about concerns that gay people are promiscuous, spreading AIDS and other STD's? I don't think promiscuity is a problem if you're careful not to spread disease; let us use the term "recklessly promiscuous". Let's see. I think the statistics on that are indeed discouraging. What could contribute to this? I've heard anal sex is like the most reliable way to transmit HIV; that could be a factor. Next, I've heard of gay bathhouses where men basically recklessly have sex with each other; what I've heard could be exaggerated, but I think they've probably existed to some extent; that would certainly explain a lot of early spread of AIDS among gay men.

Why would gay men in particular do such a thing? Well, perhaps part of it has to do with the average behavior of males; there are reasons to think they would evolve a strategy of promiscuous mating to maximize offspring, and I think young men have a reputation for being reckless. Then there is another effect: When something is made illegal but some people do it anyway, it gets pushed underground; necessarily it all happens outside the eyes of the law, and for all who participate the barrier to entry for other kinds of illegality is reduced. When drugs and prostitution are illegal, you get toxic impurities, dangerous back-alley negotiation, gang violence, pimps, and disease; when they are legal, you get careful manufacturing, department stores, and weekly STD tests. To the extent that gay prostitution and bathhouses are illegal (and I imagine social stigma can have some of the same effects as legal punishments), it is unsurprising that they lack high health standards.

Anyway, what about those concerns? The situation is unfortunate. The solution is libertarianism--legalizing prostitution--and to make people in general more prosperous, so they can satisfy their urges less recklessly.

Is a gay household an unhealthy environment for children? Inherently? I doubt it. The kids won't have parental role models for heterosexual adults of either sex. I'm not sure if that's a problem, though; homosexual role models might do fine, the one deficiency they seem likely to have (not having children) is by assumption out of the picture. Kids might find role models in the adults they meet or those they read about; it seems certainly not a worse problem than that faced by, say, single parents, or families with other problems. And note that it's entirely possible for a peaceful single-parent family to be better than a dysfunctional "traditional" family: there are a hundred ways families can be great or screwed up, and if "gay parents" is a factor, I would guess it's a small one, almost certainly smaller than "divorce".

To what extent is homosexuality associated with crimes like rape and pedophilia? I haven't heard anything about this. If there's a small link, my first guess is to attribute it to social deviancy: if you're already stigmatized and pushed into association with other stigmatized people, the barrier to entry for other kinds of deviant behavior, some of which may be criminal, is reduced. Then it's plausible that pedophiles attracted to kids of the same sex are more likely to be gay. If there is a significant link that has nothing to do with the above, I'd be somewhat interested to see it.

("Somewhat interested": if so, so what? Let's suppose childhood abuse of various sorts, the same kind that often causes people to become violent criminals, also often causes people to become gay, and therefore we observe a significant association. What then? Is the solution to outlaw gay behavior? [You bring this up in a "pro" vs "anti" gay rights debate, so I can only assume.] That seems mind-bogglingly stupid and counterproductive. Solution would obviously then be to try to reduce childhood abuse.)

Why is homosexuality traditionally considered morally wrong (e.g. Sodom and Gomorrah in the Bible), a symbol of decline (e.g. the late Roman Empire), and even outlawed? If the millenia-old traditional viewpoint was wrong, why did it last so long? I'm not so sure this "tradition" is universal. Observe the Greeks, particularly those of Socrates's time. From what I've heard, pederasty was considered great, and openly indulged in by many men who were considered great. I think they're not the only culture that was like this, although they probably are outnumbered by gay-hostile cultures.

The examples you cite appear to all come from Judaism and Christianity. Being a band of people who had to work hard to make their way, the Jews could easily see homosexual behavior (which cannot lead to children) as unproductive; the story of Onan is an injunction against refusing to father children when God willed it; an injunction against a non-reproductive lifestyle (ignoring adoption and reproduction-outside-marriage) seems not much of a stretch. So there's a possible religion-based reason. Next, I could make up reasons to do with masculinity: let's say a strong man is dominant, can only express sexuality through dominance, and it's impossible for both men in a relationship to be dominant over each other. Therefore in gay relationships, at least one man is weak, and is a safe target for bullying. (As a matter of fact, I've heard that in some cases of male-on-male rape, the "top" isn't considered gay, but the "bottom" is a gay sissy, despite the irony. The point, of course, is that the weak one is to be bullied.) The crowd gets habituated to bullying gays, and this can become enshrined in law.

As for being a symbol of decline: Well, if any behavior X is frowned upon and outlawed, and then society becomes chaotic and the law becomes weak to the point where people engage in X with impunity, then X can become a symbol of the decline, even if X is harmless or mildly beneficial. It might suggest these people have a good reason for frowning upon X, but I have examined that elsewhere.

Were the anti-gay-rights people always wrong, or has technology changed the picture (e.g. maybe STD's are now less serious due to inventions like condoms and antibiotics)? Or STD tests and general knowledge. Um... is it permissible to outlaw a behavior because many people are doing it recklessly and infecting others? How about outlawing infecting others? I guess it's usually too much to expect that kind of precision from the law. But AIDS is a very recent development; I haven't heard of homosexuality being associated with disease before the modern era. If it is, I'd refer to my previous comments on illegality, prostitution, and health standards.

Why do so many pro-gay-rights people try to advance their agenda by calling names and boycotting organizations, instead of simply patiently explaining their side of the story? I'd guess that they enjoy siccing the power of an angry mob on people, especially if they think they're justified in doing so. This may also be how they gain status in their social group.