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I haven't gotten 18, but for 16, you go back to the girls, select the dialogue about bukkake, and you select ladies!

I scrolled through the comments and found how to get 18! "For those who havent got the last story scene, Beat Dustin's novice level without getting hit, then you should unlock the final scene for act 2, hope the next acts come out soon" you can scroll down and upvote Kneets' comment so others can see it!