How do you feel about imagining your partner is someone else during sex if it makes the sex better?

For this example you love your partner but imagine the hottie you will never be with- the person from work, school, the bar, etc. and when imagining this person you are just more active in sex. Your partner likes the increase of enthusiasm during sex though they don’t know it’s because you are using your imagination.

Is this infidelity ? Does everyone do it?