You all keep saying "it doesn't harm anyone", because there is no obvious physical or emotional harm.
But I keep saying that it causes spiritual harm on the grounds that it is a misuse of the ultimate purpose of human sexuality. Yes, I know that believing this requires accepting a number of assumptions that you don't share. Thus you don't believe it to be immoral. But my view in this case isn't "homosexuality is immoral because God said so" but "God said homosexual acts are immoral because they are". So yes, I believe there is harm, not harm that manifests in an individual's lifetime (so if you don't believe there to be a life after this one, then as far as you are concerned it doesn't exist) but it nonetheless makes it harder to become like God.
But the problem is that it isn't an accepable premise if you are actually going to argue whether or not it is immoral. Yes, it will be acceptable for people who hold your views, but that's not going to work where it isn't acceptable, e.g. society as a whole. Western civilization is a secular one, one that is not based on religious law. And while religion is certinatly a part of western civilization, it isn't fundamental anymore. So if you are going to argue, in general, that it is immoral, as opposed to a matter of taste, you have to have premises that eveyone will agree on.
And that's ultimately the jist of the matter - there's a difference between a moral (something which
one ought to do) and a preference. As it is asssumed that a particular religion is a preference, rather than a fact, it is not acceptable to assume that something that is derived from your religion is acceptable as premises for your rational argument to determine whether or not it
is immoral, because it's not a given in western society.
It's the same type of thing, for example, for religious arguments against abortion or stem cell research - they may be your beliefs, and you're perfectly entitled to believe them and they certinatly can be coherant with your belief system, but it's ultimately irrelevant as we live in a secular society with a secular culture.
A moral judgment needs to be justified in order to be semantically contentful. And you can't justify a moral judgment when we are talking in a secular context with premises that are based on particular religious beliefs that are not necessary for a secular morality.