Why do some men think that body count/number of sexual Partners determine a woman's worth?

I (m20) Have never understood this and will never agree with it either but I'm wondering if somebody could explain to me, why do some men think that body count/number of sexual Partners determine a woman's worth?

I know some people personally that think like this, but they're never willing to explain it and sometimes even just call me stupid.

I'll even try asking questions like, "why is it bad if a woman has slept with 15 dudes but if I sleep with 15 women, it's okay?" And if they do answer then they'll just say "because that's just how it works".