[IAmTheMurderer]

In my point of view, a good deal of the crap that went down was Sekai's fault.
Yes, it takes two to tango and Makoto isn't absolved of guilt or any such.
However, Makoto himself seemed to be a very confused guy at the beginning of all of this. He didn't know how to go about a relationship and leaped in to the physical stuff right off.
When Sekai gives him those lessons, she pretty much does him in right then and there. He can't get his mind off of her and not wanting to hurt Kotonoha, he avoids her. It's pretty obvious though that he knows what he is doing is wrong and yet Sekai's acceptance of such behavior only encourages that.

Should he have went to Kotonoha and broke it off after he found himself in love with Sekai? Yes and he is a bastard for not doing so. However, I think Sekai should have made sure of it herself that he had broken up with her, particularly after Kotonoha kept claiming they were still going out.