that the off-fantasy of other women wouldn't bother me that much. A little, but not so much I'd go off on my significant other. it'd mostly be a pride thing, coupled with a small undercurrent of the fear in most women due to society saying things like this woman (if you're not pretty enough, your man will leave you). But I probably wouldn't do more than frown or bap him upside the head

But if it's a regular fantasy and clearly becoming "preferential", then I think here is where you start having issues in a relationship that need to be addressed (possibly by both parties)