How about a combination:
Start out as male protagonist. Sex change into female protagonist. (forced or choice..both have their own merits).
A chemical was used to change your gender, and that chemical also gave you the ability to get pregnant.
Slowly being corrupted over time is also a good option. I always liked those videos where the sister got even with her brother by putting something in his drink that changed him to a female.
The Proteus Effect, Officer Chloe, and Project Sage are some games I really liked....corruption & sex change.
And, sex change is hard to search on here cause everything gets lumped in with the transformation tag.
Have at it, game makers!