[The Giant]

Originally Posted by Jackson

I don't think it was the lack of hormones so much as the fact that he received the same treatment pretty much non-stop during his brief stint as a woman. He considered this treatment, realized it had nothing to do with his personality (or anything else about him) but rather to do with his looks, concluded that the looks and the person were, in fact, separate, and used that insight to have a look at who Miko was, aside from the hotness.

And so he arrived, at last, at the obvious: he actually hated Miko. So the gender swap didn't make Roy hate Miko; it helped give him the necessary distance he needed to realize it.

This is probably the most accurate answer, though certainly the lack of hormones didn't hurt either.