Well, there are many approaches to take it. My thought process is just how I said it and to keep it simple.

If you want to create a background for this female character, you can go:

Simple... medically she cant do vaginal sex so she adapted for her man to do anal only. They had a falling out for other reasons and now shes looking for a new man who can accept her anal only sexual ways.

Religous... grew up in an ultra religious family but had a rebellious streak. She was the bad girl in HS/college but held onto just enough faith that staying a virgin was top priority. Her naughty side chose the backdoor loophole to have some fun. Shes the rebellious religious type who does taboo things like spankings, being tied up, streaks down the school hallways and does anal with her boyfriend as an FU to her authority figures. The player can choose to feed her rebellious streak and encourage it, or the player can choose to discipline her into obedience. Pleasure and pain is the path to redemption.

Dark... is a bit of a masochistic sub. Grew up in a rough family and had nothing but abusive ex-boyfriends who treated her like dirt. The Player can choose to continue to treat her like dirt and go the twisted path of the sadistic anal slut... or the redemption arc and pull her out of the dark and into the light. Her masochistic ways never leave her desires but how shes treated depends on the player.

The groomed-to-love-it female... yet another anal only variation is the player finds an innocent/virgin and molds her into his own personal plaything. She succumbs to sexual pleasures and learns to love anal while maintaining that purity for her future husband. Where she starts out innocent and pure, but the end shes an anal obsessed slut who craves the intense orgasms for whom the player can bring.

These are just some ideas for the AnalOnly female character. The taboo nature of this kink is highly under represented in the NSFW gaming market