FutaxF - When you put on your profile that you were looking for ‘horse cock’, you did not expect something so literal, or that it would be ‘girl cock’ (Centaur/Human).

You are a woman who has always been unsatisfied in your love/sex life. Men just never satisfy you. You tried experimenting with girls for a little while, it was great the first few times, but after the novelty wore off, you found yourself unsatisfied again. The most frustrating part is you can't say for sure WHY it is so unsatisfying.

One night while restlessly trying to solve the problem, you decided to see if size was the issue. It probably was not, but it was worth a try at least. You redid your profile on one of the dating/hookup sites, telling the guys looking at it not to waste their time with you unless they had a 'horse cock'. You were being metaphorical of course, but you wanted big and wanted people to know it.

You got a reply that night, from a pictureless user. The user refused to tell you why there was no picture, except that 'he' was sensitive about his appearance. But promised if she wanted big, she would not find bigger. The user was just so confident and certain, you felt you just had to see for yourself. The two you agreed to meet in a park the next afternoon.

The twist is this a world that has life forms other than human. What you don't know if you have agreed to meet a futa centaur, one looking to mate with a human. They both are expecting a one night stand, but maybe they will both find something missing in this meeting.

This would be a deeper/long term story, an unusual purely-casual sex hookup that leads to something neither was expecting.  A world where many look down on human/non-human relationships.  Where non-humans are often thought of as lesser, they lost the competition for dominant species and are largely confined to something like reservations and limited in what work they can do.  The centaur hid her profile because she was doing something very risky.  The girl/woman (characters could be teens or adults) may or may not be more open-minded than most.

Side characters could be played by either of us, including humans and centuars, less commonly, other non-humans as well.