I play co-ed ice hockey. It's just a rec league, nothing fancy. One time I went for a pick up game, this guy had serious issue of me changing in the same locker room. It was the first time I ever had the issue come up. I come already dressed for hockey - just need to put on my pads. So it wasn't the fact that I might show skin, it was the fact that he didn't want me to see him. I don't gawk at dudes, especially this one as he was 50-60 age range. I'll admit, the college boys are nice to catch a glimpse of... back to the topic on hand, he dressed in the same locker room - I went to the restroom while he was changing. However, after hockey, he grabbed his stuff and changed somewhere else. If that's what he has to do to feel comfortable, I see no problem. My problem was when he vocalized "She isn't changing in here is she?" like I couldn't hear him.
In that case, I'd say he had a legitimate gripe. You may have arrived already dressed, but after the game, there's no telling what's gonna happen. Are you gonna just gonna take the pads off and leave pretty how much you came in, are you gonna strip down and hit the showers with the guys, or what?
And even if you're completely comfortable with that, you must realize that it's still against social norms (at least in this country) and most guys aren't gonna be okay with it. Most guys would probably joke about how much they'd love it if they could shower with girls, but I betcha if the situation actually arose (heh heh) there would be more than a few who weren't as okay with it as they claimed. And really, just changing clothes with a girl right there would bother a lot of guys.
how do these guys know the women aren't going to be affected by their partial nakedness?
If you're "partially naked" in an area that it's acceptable to be, such as the pool, beach, playing football in the park, or whatever, if the women have a problem with it, it's their responsibility to leave, not yours to cover up.
What in the hell is "partial nakedness"? Naked means you're not wearing clothes. If you're wearing clothes, you're not naked. If you're wearing a short-sleeved shirt, your arms are exposed. If you're wearing shorts, your legs are exposed.
Holy shit! Bare skin! Partial nakedness!
Oddly enough, this ties into Tricia's post a bit. There are cultural and societal norms, and they vary depending on where you are. But follow them, and all is well. In the western world, right or wrong, it's okay for guys to be topless and not okay for women. In a locker room, where it's a given that there will be various stages of undress, it's okay for member of the same gender to see you, because (again, no moral judgement here, just the facts) most people are heterosexual and it's not an issue. If you're shy or self-conscious anyway, then you are. Okay, it's a problem, but it's yours and shouldn't be anyone elses. But as others have suggested, you may be making a bigger deal out of this than it really is.