There was one series (non-H, so it was mostly played straight...) which had a similar sex change premise that got around the whole pregnancy thing with a two-fold case: Due to hormone shifts during arousal, it keeps you locked as that sex for a while, and if you got pregnant, you stayed pregnant and female for the entire duration. ...Incidentally, it ended with the revelation of that second bit. (Alongside a "Wait, you didn't realize pregnancy screws with your hormones?")