So, reading Peter as madly in love with Mary is slightly off-putting in a way that I couldn't previously articulate.

Yes, I actually feel the same about this as well. I think watching them together throughout TSG has been wonderful, a bit like student/mentor or even peers (as they are/were both Digory's students), so it's hard for me to ever picture them in a relationship that is more than that. Also, Edmund had worried about such things back in the early chapters and both Lucy and Peter had said he [Edmund] was worrying over nothing.