Currently, female genitalia have a range slider for labia puffiness characteristics, and they twitch very visibly during orgasm. Why not extend this to the backdoor: Make the tailhole spasm during climax the same way, and add a range slider to the character customizer, where left/right is width and up/down is puffiness. For example, bottom-left would result in the current "small-innie" appearance, top-right would result in a "large outie". Likely not too large, something along the lines of an equine's donut, enough to look down and see it puckering out around a penetrator. While we're at it, why not make balls twitch during climax too?
Customer support service by UserEcho