Undertail
From Create Your Own Story
This takes place in an alternate universe of Undertale where all monsters in the Underground are over sexualized and almost always horny. You will play as a human of any gender who has fallen down into the underground. (The protagonist is at least 20 years old) People can add in whatever they want as long as it's reasonable, relivent, and isn't offensive.
P.S: when adding pages add UM for male character and UF for female character before the name of the page. This is mostly just to keep track of the charater's gender and to know what parts you have during sex scenes. The two stories should remain mostly similar.