I think safety might be the most significant thing. Also, I think she's reluctant to see sex as just sex. I'm trying to encourage slow steps -- online chatting, drinks, etc. A long run-up to the sex itself.
She says she doesn't have fantasies. (We've been married for six years, together for twelve...so I'm inclined to believe her.). But she isn't turned off by the idea, although I think she might be a little bit intimidated.