In the first season, she did defend Rusty and told Monarch that the two of them were a lot alike. I'm not doubting she loves Monarch, but he is too obsessed with Rusty. (I mean, how thrilled would you be if, on your wedding day, "the one that got away" comes into your room hitting on you - especially when you deceived that one? That can't be a comfortable situation...)
My theory is that she knows that Rusty loves her, and doesn't see her as Number Two, as she accused Monarch of treating her as. To her, she may see Rusty as a sweet guy that is not obessed with a disinterested archenemy, and would treat her as an equal.
Plus, how great would it be to see Brock's reaction, since she is the only woman he's turned down?
I know a lot of people think that she's pregnant (especially since Monarch noted that her breasts seem bigger), but that is still too obvious for me, and this show is not known for going for the obvious...more or less. And Jackson Publick has stated that she is not a post-op, we can rule out the whole "I used to be a man" idea.