That’s a woosh, right? Your underwear is crawling with E Coli by day’s end?!
My jeans get washed, oh…maybe once a month. I wear jeans to work, and have three pairs I randomly pick from. Like most others, I don’t find them to be dirty unless I’ve been out to a smoky bar, or spilled something on them. In a normal day, when I’m basically just sitting at a desk, I don’t find them getting soiled. Besides, I think jeans do wear better when they are “broken in”, so I think you are doing yourself a disservice if you wash them too often.
My jeans are always tighter after I wash them. Have to wear them for a bit before they stretch back out.
Or am I just getting fatter? 
No, cotton stretches more than you’d likely guess. They’re “shrink to fit”, really.
According to Cecil in this column, “your boxer-shorts region–from belly button to mid-thigh–is crawling with germs known as coliform bacteria.”
And one of the suggestions in this column says: “When doing laundry, make underwear the last load. Don’t sort by colors (or at least don’t put colored underwear with other colored items). Use chlorine bleach, which will clean both the clothes and your washing machine.”