Is there any specific requirement that the demon's natural form has to be so squick? If it were me writing it, I would have the demon's natural form be more human, but give them the ability to take on a combat form that is unnatural to them, and very inhuman. The sheer effort you are going to have to put into making the relationship believable without creating a lead that nobody can identify with is going to be daunting, unless it's a central theme to your book (Don't judge a book by it's cover, beauty is more than skin deep, etc.)
As a middle ground, you could perhaps arrange it so that the sorcerer doesn't know what the real form of the demon is. If you wanted to go even further, the demon might not know what her true form is. It might even be that her primary form is slowly defined by exposure to others. As the years go by in exposure to him, she might find it harder to change into the squick form.
There's a lot of ways around this, but you are going to have to be careful. Beauty and the Beast worked, but the Beast was a lot more human than this demon is in her inhuman form.
