Attraction is subjective and the feelings affect it for sure, so for most people who are in love their partner will be the most attractive in the world.
The fantasy part is a little bit overboard, sure. Maybe it is true, maybe she doesn't want to harm your feelings, maybe she's just saying that in a somewhat joking way. No, her saying that does not suggest she's more likely to cheat.
Attraction is subjective and the feelings affect it for sure, so for most people who are in love their partner will be the most attractive in the world.
The fantasy part is a little bit overboard, sure. Maybe it is true, maybe she doesn't want to harm your feelings, maybe she's just saying that in a somewhat joking way. No, her saying that does not suggest she's more likely to cheat.