The kind of karma that comes with being a slut:
You do your best friend's boyfriend, she finds out and keys your Porsche.
You pity-fuck the nerd from high school, and years later at the high school reunion he's hot and wants to marry you.
Basically, instead of "What you do will come back to bite you in the ass" is "Who you do will come back as an STD" for these unlucky (or lucky) bitches.
Margo: "...and she didn't even have the modesty to cover up after I walked in on them, either. I mean, come on."
Chelsea: "She's a whore, Margo. Slut-karma will get her sooner rather than later."