Like I said, I also find it amusing at the same time. For me, it provokes a mixed reaction of disgust and amusement. I find it to be a source of comedy, and I laugh at it when I'm alone.

Sometimes, if one shows just the small tip of the underwear, that's okay, or when they sit down or bend over, like Formalhaut said, that's okay. But if they're doing it as a conscious choice for a fashion statement, to show the majority of their underwears, then it's bad, and if it's soiled, then it's really bad; either they pooped in their boxers, or they sat on something that was dirty.