Obviously there's the mage thing, but that would only work with a mage Inquisitor...
Actually, it seems to me like it should apply to any Inquisitor to at least some degree. Regardless of class, by virtue of being the Inquisitor your character does magic via connection to the Fade, and because of that connection would logically appear to be in significantly greater danger of possession or other various demonic temptations than the average person. It would seem kind of silly to me if Cullen had zero problem with any of that as long as your character's description was "warrior" or "rogue," but it suddenly became a huge issue if it was "mage."
Mind you, I'm not saying his reaction would have to be exactly the same. It may be that he could get over it more easily if he can convince himself that his lover "isn't really a mage," or that he could eventually adjust to a warrior or rogue having Fade-related powers but an actual mage would just be too far beyond his comfort zone for him to ever be able to love.
I just think it would be pretty sloppy writing if it was a huge angst source with one type of character and not even a factor with another. Cullen should be evaluating your character by what he actually sees and knows, not based on pure game mechanics. That being the case, I would hope he would at least somewhat struggle with his feelings for any Inquisitor regardless of class, if mages are still a significant issue for him.