Make a mom in CAS and then have your couple try for baby. Once you have your babies. Delete the mom. Your genealogy will still show them being related to their Dads.
Or do the above but don't delete her. Have an alien or vampire wipe her memory. It sets the relationship bar to 0 and she will not contact them.
If you like to use MCC, there is an option to "forget sim" that does the same thing. Thats what I do.
I like MCC for that reason. Male pregnancies weird me out to no end, so my gay couples happily adopt or have surrogates. I also have it set so if a female sim is abducted she will possibly get pregnant, not males.