Sex is merely a medium to show affection towards your partner. I refer back to the verses above which clearly show that God detests homosexuality. It is a sin, not because I say it is, but because God says it is. Both man and woman have the embedded instinct to reproduce by having sex. These are the same points over and over.
Sex is only okay within a marriage between one man and one woman.
I would disagree with your idea that there is an embedded instinct at work. That would make us no more then animals. We were created in God's own image not as mere animals. He gave us the ability to reason, animals don't have that ability. With that comes responsibility to act according to His law.