"But homosexuality apart from sexual satisfaction, what purpose does it serve?"
That's enough. Nobody needs to justify themselves for taking sexual pleasure in a consensual, adult relationship. But there must be more benefit to humanity, since evolution has selected for it. You've seen suggestions for how homosexuality might benefit families and villages.
Not by having sex with each other.
You responded to, "Even gay people can reproduce." Your objection is irrelevant to the issue of whether homosexuality is immoral. Being able to conceive during homosexual sex is also irrelevant. The entire issue of the moral status of homosexuality rests in it not being immoral, that is, not a harmful behavior.
Because God says so, and that is the only reason we need.
Humanists need a better reason than that somebody SAID that a god told them to tell the rest of us how to behave.
Now give us facts and a coherent explanation of the facts as to why you believe homosexuality is moral.
It's already been done in this thread by several posters here. The Humanist answer is because it's not immoral according to rational ethics, which are based in the Golden Rule and utilitarian ethics.
To be fair, the same prohibitions against homosexual sex exist in the Bible and the Qur'an. Why single out the Baha'i Faith?
We don't. The term Abrahamic religions has appeared in this thread. Skeptical moral theorists have this discussion frequently with the Christians, and they ask the same question - why are you just singing us out? Same answer.
Why would it have ever been considered a mental disorder?
Abrahamic religions demonized and marginalized homosexuality, essentially defining it as a form of spiritual sickness. Humanists are busy trying to undo the effects of Abrahamic homophobia. Getting homosexuality recognized as a natural, normal human function was one great step forward, as was decriminalizing homosexuality and legalizing same sex marriage.
Science does not deal with morality, that is within the purview of religion.
Religion doesn't do as good a job with ethics as Humanism. Humanism informed the West that secular democracies with guaranteed individual human rights was more conducive to societal happiness than the authoritarian regimes that preceded them. Freedom of (and from) religion is a humanist concept. Humanists taught Christianity that slavery and pedophilia were immoral, and are busy teaching them that homosexuality, atheism, and abortion are not.
The religious argument is that it hurts the people engaging in the behavior and it hurts society as a whole.
That's not an argument. That's a claim, one that contradicts the available evidence, and thus, while believed by those willing to believe by faith, is rejected by those who require sufficient empirical evidence (the only kind of evidence) to believe.
But that doesn't mean the God does not exist. Lots of things that were not known to exist were discovered later.
Yeah, but being empiricists, we didn't act like they were real until they were discovered. The rule is the same for gods for empiricists. Discover a god first, and we can talk, but you'll have to meet the empiricist's standards for demonstration - evidence that is best explained by positing the existence of a deity. You find that the words of Baha'u'llah meet that bar, but they don't meet the definition I provided for evidence for a deity. Those words could have been written by human beings, and support a godless metaphysics as well or better than a supernatural one.