How can something "smell bad" before it is culturally declared as such? Maybe some smells like rotting food and corpses are hardwired to be unpleasant, but probably not the smell of sweating bodies.
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?
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...