Lady Cecilia is confirmed as a mother.
If male fertility is higher, you have a reason Lord Raith doesn't just kill his sons- or nephews or brother- outright. They're breeding stock if needed.
Also, I theorize that it generally takes magic talent to become a whampire. If you have at least X talent, it's what primarily supports the demon. If you have more than that, you can do things. If you "would have" been a wizard, you are Thomas.
I think Papa Raith knows that, and it's why the Skavis plan was never tried before. It's racial suicide.