I'm thinking snail trails, or whatever guys who buy "used" panties are into.

if you want to be miserable for a year, do a pilot study where you collect underwear from men and examine them with all your senses and write down how many days you think they've been worn for ... then compare your results to the data the men provided ... I think you'll find you're extremely inaccurate. one sweaty day at work in summer isn't the same as a lazy day in winter... but as long as the guy shakes off his junk and cleans his ass like a civilized man or a gay man instead of the americans I hear about on reddit in the women's forums, they should actually be not that bad, like using a towel 2 days in a row if you actually wash thoroughly when you shower, wipe yourself off with a hand-towel to get the excess off, and air the towel out properly.