it's not rly about it being demeaning per se, but guys often want women to suck their dicks - while putting in minimal effort of hygiene, some of them - and they aren't so enthusiastic about going down on their gfs because muh taste buds and muh nose. They have unrealistic standards about how the female genitalia should be; no hair, smooth, no lumps bumps or unsightly razor marks, it should be fresh and clean with no odor or flavour, and also look nice to them, like optically.
Yeah all that comes across as extreme and OTT but most women who have been with more than one guy can attest to at least one of those things, if not all at once, in someone she's been with.
Also a lot of guys get women to suck dick and don't reciprocate, or call it a day once the job's done. Which is extremely poor communication because if it's not a discussed subject then the woman is often left there to deal with herself and the guy is happy.