Even the cleanest underwear is a little dirty...
When I was younger, I had the philosophy that life was too short to wear ugly underwear.
Now, I think mundane underwear is sexier.
Not necessarily ugly gitch, but the more boring stuff. Or cutesy panties... underwear with little flowers or cartoon bugs on it is sexier than anything lace or satin.
A thong or something tittlating with lace and bows screams "I was hoping you'd see this." But underwear with cutesy cartoon animals shows that you weren't thinking of getting any. Sex was the furthest thing from your mind that morning.
Goofy-looking underwear is worn to please yourself. To put a smile on your own face. It's not the sort of thing you envision revealing to a sexual partner. After all, wouldn't you wear something designed to generate heat in their loins, rather than giggles to stifle?
So they put something on that they thought no one else would see. Yet something happened during the day and now, lo and behold, you have before you a person wearing something more befitting a slumber party than what is soon to transpire.
What was to be a mundane day has become interesting.