On a personal note, I do not go to workout looking hot at all. I wear normal workout gear and basically sweat my ass off. So I am not at my most attractive. But this makes sense since the gym is not a social outlet for me. I run into people I know and I say what's up, but then we return to what the hell we were doing - lifting! However, I think for some the gym and a night club are the same thing. Both are places to meet people. So for them, whether they are dancing or working out, they want to look hot so they attract the eyes of others.
Some people who dress up for the gym do actually workout, break a sweat and are into it. But then again, it helps to already be built like a brick shithouse (especially if you are a women) to be all sweaty and scantly clad and still look awesome. I on the other hand, am not one of those people who look wonderful all sweaty. Although, I do look great because I get the veins showing and I'm all pumped up, but that is different.
I approach dancing the same way I approach working out. Being a former raver, when I go out to "dance" I hit the floor and four hours later I look like I did some aerobics or ran a marathon. I lose myself in the same way I do when I'm lifting. If I go to a club to actually meet chicks I tend not to dance too hard (basically, I tend to just move like 90% of most do who have no rhythm at all LOL), especially if its with a bird. Sweat is not that sexy unless its while in the thralls of a marathon shag session.