You need to crossbreed womb bacteria and futa parasite, then go to the womb and unlock the whole series of womb size and at the last one it will be futanari.
Get all the T1 characters (Total 8 of them) first, you can trace them trough the quest board.
If they didn't show on the quest board, go check unlockable symptoms or explore the human world to bring new pathogen into the body.
Breed all T1 characters then you will see new crossbreed available in your micro world base.