Anal is for Pleasure
Talk to any woman regularly engages in anal sex, regardless of whether she’s anal only or just combines anal with vaginal, and there’s a very good chance that she will say that anal sex feels better and more pleasurable than vaginal. Everyone is different and can have different preferences, but the majority of people with experience with anal sex all end up preferring anal by far.
This reinforces a principle of the anal only lifestyle that vaginal sex is for procreation and anal sex is for recreation. Because vaginal sex is generally less pleasurable, and requires additional forms of birth control to avoid its primary function of reproduction, anal naturally makes sense as the default, normal, natural form of sex for pleasure purposes.
Anal is sex purely for pleasure and intimacy without the baggage of reproduction. You just get to enjoy the best sex can be, without ever having to worry about getting pregnant or the impulses and emotions that can surround that.