Because God says so, and that is the only reason we need.
Well, you Bahai never engage with God. You don't even engage with your Messenger since he's dead. All you have are words on paper. You treat the words as if they are absolute and true, yet have no way to understand this is true in any way.
At best you Bahai should be treating these texts as general guidelines that are subject to agreement among a current leadership. Unless a God shows up in "person" and starts confirming everything the Messenger wrote you believers have no reasonable motive to think they are absolute and from a God.
Now give us facts and a coherent explanation of the facts as to why you believe homosexuality is moral.
I never said it was moral or immoral. I think it is something that isn't subject to any moral judgment. Riding a bicycle isn't moral or immoral, and I think gays having consenual sex is in the same category. The only groups who have a proble with gays having sex are religious, and these religions have existed before a time when social tolerance was not common anywhere. The world has grown up, religions have not. To my mind the prohibition Bahai has against sex they don't approve of is immora;. I think their judgmen is immoral because religions are in a position of influence and ower and they have a huge responsibility to guide people in a diverse world. And intolerance and condemnation of sex among consensual adults is no one's business. Moral positions tend to be valid when there are parties that could be harmed by other people or organizations. we can say pollution is immoral because it hurts public health. Hitting people is immoral because it harms victims. Having consensual sex among consenting adults harms no one. The religious arguments are that it hurts God. Well, no Gods are known to exist, so there is no material victim, so the argument fails. Believers can produce their God and this God can testify.
Is it because you and people who think like you say so?
We all say what we think. It depends who has the factual arguments, and it isn't theists.