Knocked Out

Jess, knowing that John would be a hope for her to get back her original body and most importantly, gender.

So Jess used her recently aquired male strength, to give {firstnane} a blow to the head, knocking John , so that she can have undisturbed sex, that will eventually allow her sperm to enter John's vagina, fertilising the egg.

That was the condition Jess was given by the mysterious phone call to allow Jess to return to her original gender. However, with Jess' condition being fulfilled, it would meant that John had failed it keeping his vagina from being impregnated, hence forcefully keeping John as a woman forever...