Playful, Dirty and Free

Why do so many people seem to see only in black and white? I'm referring to relationships and sex, the way the majority take the attitude that you're either with somebody or you're not, on or off, and that if you are with someone then to do anything with somebody else is inherently wrong. Okay, if that's the way you want it then fine. That's your choice, but really, why? Is it human nature? No. Certainly there are genetic influences on human sexual behaviour but they don't ensure rigid monogamy. It's largely a social thing, but does that make it right? Societies vary and they change. Nothing is carved in stone.