It's hard to decide what is "vanilla sex," which is what straight couples do most of the time, and what is "kinky sex," which is what some people like. Maybe it's because some sex acts that used to be weird are now seen as normal. For example, blowjobs are considered kinky because penises should only be put in women's pussies, not in their mouths. Today, blowjobs are as common as eating breakfast food at any time of the day. On the other hand, society isn't as open to assplay yet. Sure, both men and women like to see well-shaped butt cheeks, but at the end of the day, only a small number of people like to play with their partner's butt or let their partners play with theirs.