The chipmunks are changing in a locker-room and Simon takes it upon himself to observe the tighty whitie loser in the room, which happens to be the oldest of the three. Alvin is adamant that wearing briefs isn't embarrassing or for losers as they're clearly for support! Simon probably fired back that studies show that wearing boxers increases sperm count.
It's cute though. He's denying it so hard even though he's blushing and unsuccessfully trying to spin it into a positive. I guess he just can't handle his two brothers not wearing those kiddie dia-uhm, I mean, undies.