mfwb
Monogamous Friends with Benefits. .
So you're having sex with a friend but only that one friend. Generally this is an agreed upon situation by one married person and one single person. The married person may or may not be having sex with their spouse. The single person feels better if it's M-FWB. Otherwise, they'll assume you're banging half the city.
"I'm having sex with Cindy from accounting"
"She knows your married, right?"
"Yeah, it's a MFWB thing so she's cool with it"