Of course it is. You don't stop being attracted to people just because you're in a relationship.
In my man's case it's fine just as long as he doesn't mind having one of them for him. :)
As long as it goes no further. However, I think it's in poor taste if those eyes are wandering when you're out together. That's just ignorant.
As long as it's just looking---and he doesn't mind if I admire, too.