I think the check-out is pretty common and natural, particularly with guys who aren't too self-conscious about their bodies or sexuality.
Most of my friends are straight and several have commented on my package over the years. Sometimes it catches me off guard - couple weeks ago I made a joke that I hoped my basket looked good on camera for something (I was literally carrying a basket) and a friend of mine announced, "Don't worry, your basket is legendary."
My straight friends who I work out with... I don't recall any of them commenting on my cock while I was naked in front of them, but they always look (as do I) and have made playful jokes or comments about it later. And we definitely acknowledge and compliment each other on noticeable improvements from working out - in fact I'd say we're more likely to do that when we're relaxing in steam than if we were fully dressed and eating lunch. So I don't think we try to act like we're not checking each other out.
As for strangers - I check out and get checked out a lot in locker rooms. I'm not shy about being naked, but I also don't stand on a bench and do the helicopter. Even though I like sex with men I think most guys do it for the same reason I do, curiosity/admiration/envy... sometime sympathy

. But not because we want to fuck each other.