Gym

You walk to the gym, headed for the girls locker room, where you know you can always find some clothes in the lost and found. Fortunately the door is unlocked, but surprisingly there is no one around. You take the opportunity to undress and find something suitable for your current body, that of a hornier that usual, Doe. If you can't get out of her body, you can at least walk into the woods and orgasm your way back to being John. You have taken off everything and are standing nude in front of the lost and found box when you think you hear something behind you. You want this to be over soon, so you begin to slip your fingers into your slit, hoping to begin the process of multiple orgasms. Instead, what you find is a unique feeling, not quite horniness, but a stronger desire that ever to mate. Your fingers easily slip inside, too easily, and you realize what it means: you're fertile.

A voice from the past comes to you, you had always been able to achieve multiple orgasms and return to John, but there was something else to this. There was a way to permanently stay as John, but there was also a way to permanently remain as Doe, to get impregnated.