One thing is erotic games like Summertime Saga or My Cute Roommate in which the goal is clear but not disturbing or sick. Yes, in those kinds of games the only thing you have to do is to get to go to bed with women with very strange anatomies, but that's it...
Then there are games that cross the line, such as this one - Taboo Request, which is something between an RPG and a graphical adventure. This is what is called an 'incest game' in which the object of the protagonist's sexual desire is not his neighbor or a friend from high school or college, but a direct relative such as his mother, his sister or his cousin. Yikes!
You only have one mother
It seems that this drives the protagonist of this game to the edge. From the start of the game he shows himself to be unable to consider any woman to be as attractive as his mother, though he seems to know that should go no further than his imagination. However, at some point that red line in his mind disappears. If you like that, you have issues.