I don't really get it either, so I just birth them outside (Preferably at Burning Keg's Inn) to make money lmao, being greedy for money has it's perks.
Seedbed from Semen Demon is 1 to 2% Chance from other types of Semen other than Unknown, Human's, and Moot's Semen.
Or just do what I did and get Mushrooms in the Infected Forests, better chances to get Seedbed there.