First of all there is no such thing as normal when it comes to sex.
There is also no such thing as normal when it comes to fantasy. Fantasy is free space and often we don’t know where it comes from or even what it means. The key with fantasy is that is doesn’t have to mean anything beyond a thought. Fantasy is fantasy, not reality. Some fantasies people want to explore and others can remain as sexy thoughts. There is nothing wrong with you if you are fantasising about a guy from work but you need to work out if that fantasy is just a thought motivated by his attractiveness or something deeper you want to act on?
Its ok to be attracted to other people when you are happily married. Being attracted to other people is a very natural emotion and we can’t often control it. We can control however if we act on those thoughts and feelings.