“Under Christian doctrine being born with homosexual genetics would not be a sin, but engaging in homosexual acts forbidden by the Scriptures is sinful.”
Yes, that is where the morality/immorality issue comes in...if there is a homosexual gene (I seriously doubt it) it would not change the moral aspects in the least. According to Christian doctrine...or as us Christians like to say...according to God’s Word.
It teaches us to resist temptations of the flesh.