I wonder if theres either gonna be more content where ass focus and anal or even, rimming will be added, or if a mod with a focus on the booty is in the works... I dont know how to mod the game, even if I did, I have no talent in art...
if you haven't done it, Vivian has an Ass encounter. It's actually not easy to trigger on accident. You need to have a certain level of ass fetish (easiest way to raise is to loose to Tengu in 2nd dungeon while NOT IN SEX STANCE). Do the "boob fetish training" with Vivian for the Boob Worshiper Perk. If you exit your room in the Inn at Midnight, there is a chance you will see Vivian and can do a "butt fetish training" with her to get the Ass Worshiper Perk.