It depends on the situation.
I'm in a long-term happy relationship and me and my boyfriend point out attractive people all the time - he's pretty, she's got great tits, I love his hair, wow she's gorgoeus etc. Just because we're in love, doesn't mean we're blind all of a sudden. there's nothing wrong with finding other people attractive. It's like appreciating a nice piece of art lol.
However, if this guy is wanting to take it further, then it's a bit of a problem. Admiring omebody is one thing, perhaps even the odd fantacy is acceptable - come on now, we're all human. But if he actually wants to act on it then that completely anoher thing.