That was an original, if weird, setup. I love more adult yuri.
This is more my shit than, say, Hana to Hina... :P
I think that they are both masochists.
One enjoys to be hurt physically by someone who doesn't like it, because it makes her feel loved since they do it anyway.
The other enjoys to be hurt morally by being "forced" to do things she doesn't want to to someone she likes.
So the "please fuck me up" can be said by either of them in the last panel.
It should be tagged college
too, eventually, since they are both in Uni.