All tho you can't have same sex pregnancy you can still get the genetics from them both with a little cheating.
Turn on the cheats, testingcheatsenabled on, then decide who you want to be pregnant and shift click the other sim and choose edit in CAS. Go under Basics and change their sex to the opposite and click the check to exit CAS.
Now they're opposite sex so you can try for a baby. Once it takes, take the sim back into CAS and change their sex back to what it was before.
After changing the sim back to their original sex the clothing will probably be different so you have to fix that back.
Now you have a same sex couple that's having a baby with the genetics from both parents.