this is highly individual and depends on the person. to keep this simpler i'm going to operate on the assumption you're primarily asking about monogamous people. i am not personally someone who would want multiple partners so i don't really know much of anything about polyamory.
i have seen a lot of more sexual people get crushes or attraction to other people and describe it as a natural thing that happens (sometimes but not constantly) but it doesn't mean anything to them, they don't pursue it, and it doesn't compromise their relationship or their feelings for their partner.
personally, if i have feelings for someone my sights are set on them and it's basically physically impossible for me to be attracted to anyone else until my feelings wane.
in general i feel like i am a little bit of an outlier in terms of romantic/sexual people because i'm very particular about a lot of things and i couldn't tell you what romance really is because i don't think i know.