And why don't they exist in the cultures studied?
Because sex is not about union-building or having fun or for joy, even if that comes with it any way. In the cultures explored in the piece below, sex is about pro-creation, in part because the infant mortality rate is so high. And that's why even monogamy within the framework of a marriage takes a backseat to sex.
Maybe there was a time and place that such a thing was necessary and maybe that's one of the reasons certain biblical chapters were written the way they were. But we all know that sex in our culture is not just about baby-making, even the ones who try to pretend otherwise. And our infant mortality rate is not so high that we have to implement these sorts of strict beliefs about sex and masturbation. People misapply lessons from other cultures and other times when they insist that gay people in committed, loving relationships are sinful.
The same thing happens in our society in a variety of other ways, lessons being misapplied. The same thing happens even with the English language. Some overly-strict grammarians misapply rules from one language to our own and have determined that things such as prepositions at the ends of sentences should always be frowned upon. And they insist upon such rules no matter how many times linguists debunk their concerns. There is no one right way to speak or write the English language, despite what many people say and believe, just as there is no one right way to love, no matter how frequently some insist that there is.