I have wondered about this in the past myself, my girlfriend and eventual wife says I'm the best boyfriend ever too.
I think it does have alot to do with the fact we accept there bodies when others ridicule and dislike them for it, or the fact we don't mind AT ALL if they gain weight or lose it. To me there happiness is more important, which in turn gets me kudos from my girlfriend.
I can say that being an FA is good for my relationship with my girlfriend, I can give her support and comfort when needed and give her actual positive critique on clothes and how they look on her with out letting it get sexual.
In general I think men who are in tune with what a woman needs and wants and who can look at the negatives and positives of the plus size world and who are FA's are generally better boyfriends. But they also have to not want that lady to change or be someone else they don't want to be and then in turn be an ass about it.
All I know is that I'm an FA and love my girlfriend for who she is and how she looks, her having a very curvy body is just a plus to the whole package that is her. She loves the fact that I love and respect her and her body and that she doesn't have to worry about me not liking her body or finding her unattractive.
NEVER going to happen.