You make a Will save with DC equal to your pleasure every round you're tentacle raped, failure means you take Ahegao damage and your pleasure goes down some (probably down by 10 or so, not quite sure how I want to balance that part; might be 10+Will save bonus or something).  Normally, pleasure goes down by 5 every round if you're not getting raped; with that plant's nectar, it doesn't drop on its own for several rounds (it does go down if you fail a save, though).