As I understand it. Incubization only lets you feed on their demonic energy as a source of food.
Mamono can't drain you dry even when you're human because they replenish your mana instantly, which is why love is required since it will increase the total amount of mana given and recieved by alot and your mana limits will slightly increase until you become an Incubus. As mamono sex without being married gives them next to nothing because they give it back to you.
This system should make it impossible for a mamono no matter what to kill you by draining your mana via sex or other means.