Of course. It's no big deal--an attractive person is an attractive person.
I've read scientific studies that state that guys kind of can't help it. It's just the way they are wired by nature. It doesn't mean they are interested in cheating on their SO, and it doesn't mean that they are even comparing the person they are checking out to their SO. It's more like a, "oh hey, look at that pretty butterfly!" kind of thing. It just turns their head. Really nothing to get upset about, unless he's being a dick about it. If you and other people in the bar were all noticing how he was checking her out, he probably wasn't being to subtle about it. Tact is nice.