This is an interesting premise. The Redo of Healer universe has the concept of 'sex = mana transfer for magic purposes' in there, but in the case of power gain, I'm sure Keyaruga would have a ball of a time 'boning' Mamono to obtain magic from them. Imagine if he boned a Lilim - he would have intensified his healing magic a thousand-fold. Tho that's just me thinking on a surface level.
-Jeiel Yozama, thinking tritely,