Your case is "if this is just how they've been wired biologically, is it wrong?" The answer is their biological creation isn't wrong, but how they respond can be depending on how they do it. If it's a response that does interject what God has in mind, then it is wrong. We can't logically have a right answer that differs from God. If a gay guy has feelings for another guy and intends to save himself for him, marry him, and live faithfully, the sin he has committed is disobedience to God's model of marriage. His intentions may be faithful, but his faithfulness to God has been corrupted only because it sidesteps the holy definition of marriage.
It's not a legal argument or a even an argument of kindness, but it's really a theological one. Marriage isn't just a physical institution, more importantly, it's a spiritual institution.