And so?
Leaving everything else aside for a moment, just put what you're saying in another framework. I'll re-word what you said:
"I can imagine that with hard work and discipline, married men can change their behavior. But I'm dubious that their basic orientation can change. As a heterosexual male, I can't ever imagine ceasing to find other women attractive. Why would it be possible for married men to cease finding multiple women attractive?"
Then put the same thing in the framework of being attracted to children, rape, animals, what-have-you.
Everyone has desires; everyone has wrong desires. How do we know they're wrong? What do we do about them?
That's the central question, the central issue. There are basically only two answers: (1) we can't ever know, so we make it up; or (2) God said.
Or 3) Because they’re counterproductive to our lives.