Oh, so I guess it might be more of a performance anxiety or insecurity issue.
Try to think of it like this instead of taking it personally: if fantasizing before/during sex helps her reach orgasm quicker, harder, and maybe multiple times, then what's there to fuss about? She's cumming and you're the one making her cum. I guess there will be girls who may leave you for a gang of black men or a dog due to her inability to be sexually satisfied with you, but at the end of the day, if you can give her good orgasms then she should be happy.
I'd say you're in the best position if you can allow her to openly express such fantasies, and maybe even help her out with it. Does she like BDSM? Tie her up, spank her, and make her cum hard. Does she have gang rape fantasies? Buy another dildo or two and fuck her hard (sounds difficult lols). She'll have a hard time finding another man who's so open and accepting as you in today's world of super friendly, shy, and supplicating men. Oh, and what if she isn't satisfied with you and it causes problems for the two of you? Replace her.