I am having the same issue. Whenever I go into Cas to plan/change outfits or any other CAS parameters, my married sims can no longer perform romantic interactions, being treated by the game as if they were siblings.
I tried planning their outfits from manage worlds instead of accessing CAS through live mode, but no difference.
Tried vanilla (without mods) and with mods, no difference.
Found a somewhat feasible workaround that doesn't require you to make your sims forget all their relationships, or replace them with "clones" from the gallery and lose their milestones etc.
Move Partner A into a different household, change them in CAS there. Move them back.
Move Partner B into a different household, change them there, move back.
Do the same with the child.
As soon as one partner + their child are still together, it removes the romantic possibility between both parents.
