At the moment you can collect semen only from those 2 characters. More characters will be added in later updates.
That quest is still under development.
Sounds like you've defeated all the bosses so far.
To get through the big mushroom futa, you'll have to wait for a later update when the blue mushroom girl is added. Once she's added and you've defeated her, you'll complete the big mushroom futa quest and can progress to the mushroom village in a later update.