You have to do a few different things to lower purity as each one that does lower it only has a set effective range . This is the route I used to get corrupted Lance. First the kiss/ride that horse for the initial corruption, then jacking off in the evening with high arousal, then going to the bar will lower it further bit by bit, after that stops lowering it go to the prostitute with high arousal that'll lower it some more, finally catch the bandit and urge Lance to fuck him this one only works/appears when you either use the top group's choice to find the bandit or when Lance encounters the Bandit of their own free will.