what.
sex is a terrible way to mana transfer. if you summoned a creature, whatever contract you signed with them should probably act as a medium for mana. if you're giving another human mana, either they should know how to receive mana(caster), or just not need mana at all(non-caster).
and in the same world where mana transfer is physical, we've got massive fireballs and intricate rituals going on.