crushes are fine. being full on in love with more than one person is fine. itll happen to most people at one point or another. things only get murky when you decide to act on these impulses. you need to consider whether its worth giving up what you have and ruining another couple's relationship just to act out on something that might shortly fizzle away.

you could also, of course, try to keep it a purely physical fling. more difficult than it sounds, as pesky emotions almost always complicate this, even when you would otherwise get away with it. wouldnt recommend it, as it tends to become a poor habit thats hard to shake and will simply complicate your future relationships.

this is also assuming he has any interest in you whatsoever to begin with. if not, fantasize away and be content with that.