In fairness, their brief time dating was portrayed as deliberately fucked up.
I'm not that familiar with that part of the
Ultimate Series (I've really only read the
Death of Spider-Man and it's prelude from the post-
Ultimatum Peter-centric comics so far). So, I could be missing something, but my main thing was (besides the fact I was rooting for Mary Jane), that I'm not sure that Peter dating Gwen really seemed to offer any story points or explore the characters in a way Peter's previous two breakups/reconciliations did.
One thing I do kind of appreciate about that little story thread is the way it kind of forces us to reexamine some of the earlier material. From the pre-
Ultimatum comics, we do know that MJ was initially convinced that Gwen was in love with Peter (implied in the
Double Trouble trade paper back and confirmed in the
Public Scrutiny trade paperback). Gwen herself denies the theory in the
Carnage story, however, I was under the impression that Gwen initiated things with Peter post-
Ultimatum (at any rate, when breaking up, she seems to think she manipulated him into it, which suggests that she had approached him).
So, those few points raises an interesting question: Was Gwen lying to MJ in the
Carnage story about how she felt? Gwen emphasizes the point that Peter clearly has eyes for MJ only, so it could be possible that she realized that any feelings she had were unrequited and so it wouldn't do anyone any good to say that.
I can't prove that (the only real details we get about Gwen's thoughts on her and Peter was that she couldn't tell if she loved him in a familial or romantic way and also realized that MJ was the only woman Peter was truly in love with, which would be the case either way), but it's interesting to think about.