@engineer,
I actually don't agree with that, engineer. Or more exactly, I believe that way of thinking is deeply bound into oppressive and arbitrary notions of sexuality and family arising from, mainly, the western world's christian heritage. Other cultures view these things much differently.
For example, there is a particular Asian culture where homes are designed with the wife's bedroom having one door into the main house and another door directly to the street. The wife sleeps in her room and the husband in another. At night, if another village man knocks on the wife's street door, she can invite him in if she wishes for sex. The husband, of course, can go out on the street and knock on some other door. (reference here is "Sex at Dawn")
One is constrained by the culture one is in but even though that is true, there is great variation in any population as to sexual behavior. And in my view, pushing back against arbitrary cultural fixed ideas is a good thing.