I don't think that was a pity sex as /a/ tries to imply. And here's why:
1) Kansai wants it all. She wants MM as a boyfriend and wants Loli as a friend. She's just can't decide who to sacrifice in this situation.
2) The sex was awful, i know that but it was nor because of pity. It was because she was feeling guilty about whole Loli thing.
3) She just can't betray Loli. We seen it before, when she stayed up all night with MM just to cheer him up. It wasn't because she wanted to spend some time with him, she did it because she's a bro. That implies that she cares for her friend more than for herself. And why it should be different in this situation? She chose to sacrifice her own interests just to gain time to make up with Loli.
In the end she'll end up with MM no matter what. And it is because she loves him. Disclaimer: I haven't read spoilers, it's just my opinion. Oh, and sorry for my bad English.