Heh. I've asked people, and I can tell when I smell bad, I do occationally but I rarely get BO in that sense. It just smells like clean sweat, with whatever that scent is that each person has that you can't wash away. So if you don't like the smell of sweat, sure then maybe you consider me as smelling bad (when I'm sweaty...I still take showers daily, sometimes twice a day after I work out though it has more to do with feeling clean rather than smell) but I rarely get that BO smell or whatever it is that is nasty, I can't describe it but I can tell when I get that.
Secondly, I change my underwear every day, regardless of if I worked out.
And I don't get how some people feel the need to change their shorts/pants every day...mine just don't get that dirty/sweaty.
Oh and not all sweat produces bacteria, there are several conditions that prevent bacterial growth, pH levels being one of them.