Sex that's characterized by its unoriginality, adherence to routine, lack of exploration, conventionality, and obstinance towards anything new.
Typified by the near-exclusive use of the missionary position with the most adventurous activities being the most modest diminutives of rear-entry position (accompanied by boisterous claims about said ventures, ignorant to the humor with the actual lack-of-adventure involved), and borderline-repulsion to (and a passive-aggressive, condescending judgemental attitude towards people who like to enjoy-) all things ranging from the extravagant, like group sex, to the moderate (like anal sex), and, often, even to ones as benign as oral sex. Oftentimes accompanied by low sex-drives that result in sexual encounters occurring at low rates, like once every week (if not less-often than that.)
It's not necessarily bad if that's all that both parties want, but it is a limiting preference and the judgement and repulsion many of these people have over those who desire more is not an admirable quality.
Rebecca: "She did WHAT with Lebron? Eww. Why would she do that? That's gross."
Lamar: "Shut the fuck up. People who only have vanilla sex like you don't get to have an opinion about sex."
Rebecca: "I... am NOT vanilla!"
Lamar: "Yes you are. You ONLY ever have sex face-to-face, man on top, and always talk about that one time you did it from behind while you were spooning with Tyrese like you did some crazy thing. And you get disgusted any time people bring up fucking blowjobs or pussy-eating or anything, and you use that one time you blew Durrell for 3 seconds as proof that you know what you're talking about. And you only have sex, like, once or twice a month, at best, and you act like that's enough for anyone and get offended every time DeMarques wants to have sex more than that. That's the definition of vanilla sex."
Rebecca: "Okay, first of all, me and Demarques have a very satisfying--"
Lamar: "-- DeMarques hates your sex life."
Rebecca: "Oh really, how do you know?"
Lamar: "Oh, I don't know, cuz' he fuckin' TOLD me."
Rebecca: "So there's something wrong with me wanting to do what I'm comfortable with?"
Lamar: "No, but acting like other people are fucked up for wanting more and suppressing your disgust every time someone brings up sex that's not your bland nothingness-- like they're degenerates for not wanting to have bland vanilla sex like you-- is insufferable, and your relationship's gonna end because you're not willing to budge on this."