Morality or... stinkiness... isn't the real reason why Orin shouldn't be romanceable. Pathfinder games already made this pretty clear with, say, Camellia. And, well, this game has Minthara.
The REAL reason an (at least, conventional) romance doesn't suit her is that it's not her thing. I don't see why she would care about that. It probably doesn't even register for her. And even with sex it's probably something quite twisted and somewhat devoid of sexual aspects, if she even has any. The idea I mentioned could work since it's more for trickery reasons and general Evilness, but a more conventional affair/romance just isn't very in character for her.
