This is probably more of a later-in-the-series question, but - any chance Cat might find herself attracted to another girl, or has she sworn off women entirely? I admit, I'm thoroughly tickled at the idea of Marius (after the requisite lecherous leering) finding himself in genuine competition with a member of the gender he reflexively objectifies. :)
Cat hasn't sworn off women any more than she has sworn off men. Up to this point, relationships haven't really worked for her regardless of who they are with.
While I love the idea of Marius squirming between wanting to join Cat & another woman vs wanting her all to himself, I don't know that I'll take the story in that direction. It doesn't fit with the arc that is in my head at this time.