I don’t think the question is whether fantasizing about someone else is “right” or “wrong” (and anyway, what could those terms possibly mean?) but rather it indicates perhaps there are ways to connect that aren’t being utilized. Too often, in my opinion, sex lacks the mental intimacy that results from shared fantasies and memories. A few standard words (“slut, dirty, whore, fuck, shit”) are the sum of verbal interactions plus a few words of encouragement (“don’t stop, that’s good, oh yes”). But at least 75% of sex happens in the mind, so when we don’t use everything at our disposal to connect our minds it’s not surprising they’ll drift elsewhere while our bodies are busy with the physical stuff.