honestly, heather. luke is motivated mostly by romantic attraction and valentine has been in the game so long that she's just kind of over it. since theyre both succubi they also dont experience horniness like humans do, when they go too long without sex they start feeling weak. but since heather is a human who was single basically her whole life until luke she's been pent up for years. if luke hadn't been physically draining her energy in horny 2 she could have kept going like that forever lmao