Sex is necessary for the continuation of species. That is why nature has built in our genes the craving for sex and frustration when denied of it. This is nature's way of protecting and perpetuating the species. Otherwise, people may lose interest in sex and may not procreate.
However, everyone sleeping with anyone will make us only as good as animals. So man has arranged the way of life under "one man, one women" principle under the society's blessing called "marriage"
Even then, man is still another animal and sometimes the ancient beast comes out
