The fun path is where you do both. One of the girls responds better to slaps, she Likes the pain. But you can't just go straight 'rude' path with her, you have to vary it up a bit.
That's the problem with the 'auto' function for each path, it removes the potential for the player to choose the 'optimal' path through the story, and it misses a lot of content because of it. It's not best to be straight 'nice' or 'bad' all the way through, you have to do things that are appropriate for the various encounters. For example - Felicia, Tegan, and Hannah respond better to nice options, Hannah wants your attention but you have to get a bit flirty and pushy with her to get that part, and Chloe likes being abused - she wants the pain, but she also wants to be treated nicely sometimes too, but responds well to a 'firm' hand.