I am making the assumption here that you will agree that God has given Man a few moral laws, including a prohibition against homosexual acts.
So, on that assumption, and realizing that all mankind, including even Jesus, are subject to temptation, Free Will is the ability to act on those temptations and sin, or reject them.
So even if homosexual lusts are genetic, or occur during the development of the fetus, or in events happening in early childhood, we have no more excuse for giving in than we do for any other type of fornication, also forbidden by God to homosexuals and heterosexuals alike.
Now, there are other ways to argue against homosexual practices, but Free Will is a fundamental religious concept, so that is how I choose to argue it.
The same argument would apply to any other sin, or moral decision. We don't have to give into our dark desires. We may not be able to keep from having them.
Heck, read Martin Luther's De Servo Arbitrio and other works for that topic.
As a character of Robert A. Heinlein said in The Moon is a Harsh Mistress, "So Mike [a computer] started with 'free will' and acquired more as he was added to and as he learned--and don't ask me to define 'free will.' If it comforts you to think of Mike as simply tossing random numbers in air and switching circuits to match, please do."
As I have stated repeatedly, I'm commenting on this case, where a student wanted to get a degree without completing the academic requirements and refused to acknowledge that professional sessions should be conducted in accordance with the ACA Code of Ethics.