You seem to define natural sexual behavior as some sort of subjective concept that differs from person to person depending on their preferences or personal attractions.
According to your definition, if a person enjoys sex with animals or dead people, we could not call it "unnatural" since it feels natural to him.
I disagree with that.
It am not refering to what "feels natural", or an individual "orientation" but rather to the "natural function" defined by design and God's created intent for sexuality.
The Bible's stand from begining to end..is that the only "natural" sexual relationships are heterosexual, and that all homosexual relations are "unnatural".
That is what I believe.
Your response doesn't answer the question of why would a man who is naturally attracted to women freely engage in something as painful and unnatural as anal sex with another man instead of pursuing a woman. That's the one question none of the people who feel it's purely environmental have seriously tried to answer.
They won't? Why is it that the "environment" of prison is known for producing homosexual conduct among men who claim to be hetero?
Why do hustlers and hetero men exploit homosexuals for money?
No one can answer all of these things conclusively but we know they involve both environment and choice.
Some African cultures practice "dry sex" - women take various alarming steps to dry out the natural moisture in order to please their men.
And besides, if you've never tried it, how do you know it's painful? Ever heard of KY Jelly?