I think a lot depends on longevity. It would be pretty silly to expect a partner of a relationship going on for a few years to never fantasize during sex. When things are new then sure. When monotony of years starts to set in then it's an unrealistic expectation.
He shouldn't have told you about this though. And you shouldn't have asked him.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~