Peer pressure is the mechanism by which it's introduced. Once introduced a child can either reject it wholly or gradually assimilate it into who they are. You have to remember: Children's minds are very malleable when young. You can get them to believe almost anything or do almost anything if they're told it's ok enough times or if they see it enough.
Think about it like this: When a lot of folks are young, the mere thought of oral sex is fairly disgusting to both sexes. Unless it's introduced to the individual, typically via peer's talking about it, joking about it, "you ain't never had your face in a chick's pussy??? nigga you don't even know what you're missing"-type shit someone could conceivably go their whole life without ever doing it. Same for anal sex: There are women that tried it and hated it, tried it and eventually warmed up to it, or tried it and loved it. But prior to trying it they were prolly on some "eeeeew, hell nawl" shits but tried it due to pressure from a group of their peers or a guy they were dating (also a peer). Likewise there are men that wouldn't dream of sticking his dick in a woman's asshole. Let him get with a woman that brings it up multiple times over the course of their relationship and he just may try it or if his friends are all talking about how they've done it and liked it he may be willing to try it.
It works like this with pretty much everything when you're a kid. Get enough people around you to tell you that XYZ is ok and you'll start to believe it yourself.