Not always. Granted, I've had VERY limited experience, but I don't recall "checking out" other women when I was in a relationship and pretty happy. I know of a select few guys who have said the same thing, so some of us aren't wired to look.
There's also checking out, and then there's full blown oggling. Now, if you're a man who's in a happy relationship, you can look at other females, but have class about it, The old "LOOK BUT DON'T TOUCH." Then there's being a jerk about it.
Personally, I'm a single guy, so I can check out any woman (I try not to, though). But honestly, a lot of the time, I'm not thinking about "nice ass..." It's more of "Wow. How can I talk to HER."
Honestly, we're just curious about some of you. So, I will watch what a woman is doing.
I have a crush on a certain woman, and she's admittedly gorgeous. A friend of mine and I hang out where she works. One time, she had her back to us, she was cleaning and fixing something. Now, I was watching her, mainly just trying to figure out what she was doing, but also trying to figure out something cool to say or if I could help (score brownie points, eh?). My friend was staring at her butt.
Here's my explanation. There's one scene in the movie "Some Like it Hot" - In the movie, Jack Lemmon's and Tony Curtis' characters are supposed to be pretending to be women (to avoid the mob). They get a job out of state with a woman's band, so they try to flee with the women's group.
This is the first time they're in drag (hilarious), and trying to catch a train. They see Marylin Monroe's character run past them, and they're both looking at her. It goes from "look at that" to trying to figure out how they walk without looking ridiculous. "It's like jello on springs! Must be some sort of built in motor or something."
So it's not always "just the lust" that we look. Sometimes' we're in the habit of trying to figure out what you're doing or how you're doing it, or WHY. Sometimes it can easily be mistaken for lusting, but honestly, it isn't always about that. And I'm being honest, there are some guys out there who don't check out other women when they're in relationships.
Now, it's your job to figure out if you're being unfair to the guy, or if he's being a little too obvious and crossing into that innappropriate/rude category, the best way to respond from there. I can't tell you that. Good luck.