So I know that if you suck a bull dick at night it increases your corruption but it is not allowing me to do that. How do i get the option to suck his dick 

i believe you have to become corrupted and to get corrupted you need to let the slime have sex with you but i think 100 percent corruption ends the game

You need to lose in the battle of the slime 5 times to get the COR point to 5 then you can suck his dick