How to tell my fiance to wipe better?

I recently did my fiance's laundry because his washer is broken. All was going fine until I started working on his underwear. To my horror, there were "skid marks" on every pair. How do I tell him he needs to try to wipe better? I don't want to wash crappy laundry (at least not from a grown man) for the rest of my life. Thoughts? I don't want to hurt his feelings or make him feel dirty.