The replacement for synthetic underwear is wool, not cotton. For weeklong wilderness backpacking trips, I used to vacillate between a fresh pair of synthetic boxer briefs every every day, which took a lot of space, and one for every other day, which was gross. Now I wear a single pair of wool boxer briefs all week, and it feels and smells better at the end of the week than synthetic underwear does after two days. Same for sleeping: I used to bring a couple of pairs of synthetic boxers, and now I'm down to one pair of wool underwear. Two pairs for the entire week, one for days and one for nights.

Granted, wilderness backpacking has completely different standards for smells and grossness, but the comparison carries over to the higher standards of the "front country." In hot, sweaty weather, synthetic underwear gets noticeably grosser and smellier than wool over the course of a day.