In the 20th century and forward Anal hasn't been as small of a group as it was earlier. It got wider and wider as people started to use the backdoor to give pleasures to their partners, let it be male or female or tranny counterparts. Becoming so widely accepted and pursued, from being a small inner fetish/taboo circle of people doing it, it gained the popularity it needed. Anal is kind of how porn evolves people from being grey and uneducated about their bodies, being useful in acceptance and everyday life also. If you never fingered your wife's ass while she sits on your face, you are missing out.