I find the amount of 'slut-shamy' feelings I get (towards both myself and others) increases when in the presence of men who view sex as something that makes them more valuable, and decreases when around men who seem to not care.
It's so hard to pin down, but some men give off this vibe that they want sex with me because it would be validating for them. This vibe can occur regardless if they see me as high status or 'just another number', and regardless of if they're playboys or not.
When I'm in a culture where sex is validating for men, my subconscious brain screams NO when I see other girls have casual sex, like they're stupid or being duped. I try not to think these thoughts.
I've long felt that for guys, slut-shaming is about guys shaming women who won't have sex with them but will with others. A slut is the one who won't do it. Not sure about women slut shaming other women, I think it may be about status and social control.