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.
That was, what I mean by gets off by killing people. That is literally a thing with Bhaalists and I doubt, that a conventional romance would do it for Orin in any way. Her romantic partner would not survive a day or night in her companionship.
