And genes are not the only thing that can result in an inborn trait. There is strong evidence that hormone levels in utero are a key factor in homosexuality.
I think you're just choosing to ignore the evidence, but neither here nor there.
Alcoholics tend to harm those around them. I've yet to hear of a gay person doing so, simply by being gay. I don't think the two compare.
Regardless, since your opinion is religious and you live in a secular democracy, public schools have every right and indeed an obligation to teach a neutral view of homosexuality.
If you don't like it, home school, private school, or move to a theocracy. But imposing religiosity on the public sector is quite clearly unconstitutional in the country in which you live.