Have you considered body part growth over time along with contributing boosts from active use in addition to more direct, faster options ?
I could see it working something like the following:
All genders have passive hair growth over time.
Males get penis growth over time.
Females get breast growth over time.
Herm's get both breast and penis growth over time.
Works something like a passively growing experience bar - fills up a little bit say every 30 mins of active (not paused / in combat / etc) game time and once it hits 10/100/1000/10000/100000 (bigger sizes need more points so the character does not go from a flat chest to enormous breasts in a couple of hours)
The body part in question being used for something gives an immediate increase to the experience bar - getting 'milked' contributes to the growth experience of the body part in question - in the same way that striking something increases strength.
Stick it behind a toggle for anybody who does not want it - could be a more in depth toggle allowing the function to be disabled outright or disabled when the character reaches a maximum size.
Another idea while I'm thinking about it:
A way to change a characters gender after the game starts - some sort of fast acting potion / drug / medicine as a more permanent option (until a another consumable is used) and a spell as a temporary option - I could see this as something available at the succubus tower and maybe at the science lab.
Another one to stick behind a toggle I would think.