Sarah wants to sit on Mark's face while watching TV and of course Mark will serve her as a human seat pad. She's wearing a tight pair of jeans and doesn't care about the pain she inflicts on Mark's face.
Becky and June take turns sitting on a man's face while they chat and enjoy a cigarette. They don't care about the man under their asses. He doesn't mean more to them than any other piece of furniture - or probably even much less! Natural purpose for a loser like him!