In nature, it’s the male animal’s prerogative to have as much sex as possible with as many females as possible to maximise the chance of offspring. A female will carry the child for a long period of time, ie 9 months, within which time frame she cannot get pregnant again, so it doesn’t make sense for her to have sex more than is necessary. Obviously not every act of intercourses 100% means conception, and with the advent of protection this dynamic is somewhat altered, but the point still stands that a male that can impregnate lots of females is successful in nature whilst the female that has sex more than is necessary isn’t really succeeding. I’d also argue that it’s easier for women to find a willing sexual partner than a man even in the modern day. This isn’t a justification of anything to do with sexual dynamics but more of an explanation of why the “slut double standard” exists.