To be fair Valeria isn't pregnant either so i wouldn't say the mc has super sperm.Thing is, MC finishes inside Cass three times now, and she still isn't pregnant - since he has super-sperm and knocks up ladies from the first encounter, we can safely assume that Cass is using some method of birth control for now.
Cass will be pregnant when the story allows it. Especially as she currently has too many variable states as is, adding pregnancy would further dilute her routes.