What makes you think the smell of sweating bodies can't be hardwired to smell unpleasant? It's the result of bacterial activity. Can your nose tell the difference between the bacterial activity in this case, and in more harmful cases?
Because smells are not hardwired [1], we are nurtured into being distasteful of the scent of sweat. One nice example of this that I like is that you can find infants happily playing in faeces without being disgusted by it, it doesn't bother them the sligthest.
[1] http://www.foodnavigator.com/Science-Nutrition/Nurture-plays...
The fact that people forgot about it and get used to it pronto, as you already admitted above.
Also, who said people found the smell of a sweaty human unpleasant in first contant (before olfactory fatigue)?
This pressuposes an essential "badness" of the smell, regardless of if someone perceives it as bad or not. Where would this "badness" essense lie?