Don't forget that he produces enough mana to keep A Ram Demon satisfied by himself and she is nothing more then A party time lover to him. Vails mother wanted to test him because she was curious too see what her daughter was still in good health with having ju st 1 lover .My personal theory is that mom is some sort of Mythic like a siren. The MC's near-supernatural ability to seduce and sleep with nearly every woman he meets with barely any effort is his own Mythic powers manifesting - and being the half-breed child of a Mythic and a mage has basically left him more than the sum of his parts (which is why his powers manifest subconsciously in spite of only being half-Mythic).
If the MC ever fully awakens his powers and can use them at will he'd basically be almost god-like (hence why there's a bit of awe over what he is in Neferi's storyline), but even without that he just exudes irresistible charisma and draws other Mythics (at least the female ones) to him.