A lot of people say that vanilla sex is called such because it is bland or without “flavor”, and they normally use this term to describe the missionary position or its kin. What they probably don’t know is that the word vanilla derives from a Latin word via a diminutive form of the Spanish word vaina, meaning sheath as in that’s what a vanilla bean pod looks like.
What they might not realize is there’s another word deriving from this same Latin root, and that of course is vagina, which literally means scabbard– i.e., the sheath one puts one’s sword into. Ergo vanilla sex is etmyologically appropriate and the blandness is just a coincidence.