Why don't men and women accept that ual promiscuity (a.k.a. cheating) is nature and is not really a choice?

Hi all! In our US society, unlike 50 years ago, we now accept premarital and cohabitation with a partner but we still don't accept adultery/infidelity. Why is that? I think the first two things I mentioned are much more worse than cheating on someone. Why don't men and women accept that ual promiscuity (a.k.a. cheating) is nature and is not really a choice? Yes, we do have the power to choose whether or not to have with multiple partners but the instinct in itself is natural by default. If it wasn't, people would'nt still be sleeping around after billions of years of human evolution. That's all I'm saying. What do you think? Any answers would be appreciated! Thanks!