You're not wrong but that's not the full picture of the problem. Why shouldn't a man who is desirable by most women indulge himself in having sex with many different women?

Because doing so is selfish and harmful to the community as a whole because each woman he does that to becomes less attractive to other men; thereby, making it harder for the community to grow and prosper. It negatively impacts the happiness of all; however, men will still be selfish and attempt to do what is only in his interest (make mistakes). It's for this reason good communities would then encourage a man to marry whoever he slept with and most especially if he got her pregnant.

It's not just on the men alone either, women should not be giving it up easy to other men for the same reasons and because it does have a negative impact on her value as a woman.

We've completely lost touch to these things as a society.