Ideas
I’m disappointed that this has so few votes because I find it really upsetting. I have two male Sims, one of whom has a daughter from a previous relationship, and I really wanted the other one to adopt her when they got married. I’ve tried everything: cas.fulleditmode, saving the household deleting the child then re-adding her from the library, MCCC (which just completely messed up their family trees). As a queer person this feels like a slap in the face.
Edit: I found a workaround with MCCC! It’s a little convoluted: I changed the gender of one of the parents by clicking on them > MC Command Center > Sim Commands > Maintain Sim > Swap Gender (this will change the gender without changing their clothes or appearance), then I went to MC Command Center > Relationships > Add Relationship > select child Sim > select Mother or Father. Once both adults were set as the parents, I just swapped the adult’s gender back and now the child has two fathers!