Could be you have an unhealthy addictive attraction,
but also, what if you just aren't happy or feeling attractive and beloved
in your current relationship and it's gone on for years and years?
Is feeling desire for another or being desired unhealthy then, or a longing for health?
Sometimes the heart tells us what the head doesn't want to hear.
I'm not saying this young guy is anything real, but maybe he is symbolic
perhaps of feeling loved, or paid attention to?
I think there is unhealthy obsession, and also true feelings wanting to be expressed.
It's hard to know which one is going on at times.
I would never advocate extra-marital expression of such things,
but perhaps you need to explore these feelings in therapy?
Sometimes marriages run their course. People change.
Other times, you just obsess on other people to escape your own internal issues
and like Anvil said, you are escaping your own issues in fantasy.
Sometimes, the marriage can healed with hard work on both sides, sometimes not.
The first cause of external attraction (unfulfillment) deserves serious attention, and so does the second (obsessive escapism).