Again the guilt is biologically ingrained. For example, why would anybody feel guilty if they had sex with their mother or sister ? If two adults consent to have sex with each other so what if they are related ? Yet incest is taboo, because inbreeding is so dangerous. And using birth control during incest, makes little difference in ameliorating the guilt. So with issue of "sluttiness". the fact that you are single now, yet had sex some months ago, may make you feel guilty to start a relationship again. Why? Because in nature women can get impregnated months prior, and when someone else comes in they may start spending resources on a baby that doesn't share their DNA. So biologically speaking, the woman cheats that man by burdening him with a baby that it's not his own. Hence the guilty feelings, and the condemnations from other. And since they didn't exist in our evolutionary past, birth controls are irrelevant when it comes to the arising emotions. There is an evolutionary truism that is's worth remembering: "Sperm is cheap, Eggs are expensive". That is a man can father, at least theoretically, thousands of babies. While a woman is restricted to having a baby or two once a year, plus subsequent care. Based on this, there is an inherently unequal dynamics between men and women. Of course, because it's natural doesn't mean it's desirable. It's just we have to know what instincts drives us, to better understand the source of our happiness and unhappiness. And perhaps confronting the sources of our guilt, may break at least partially their spell over us.