Best definition
Xylophone Sex
to line up five or more females by either height or pitch of their voice, and proceed to bent them

over and have the most amazing Xylophone Sex ever. (if done right they all will make a different sound during the Xylophone Sex)

Jim: dude i just had some great Xylophone Sex in the band hall!

Bob: holy shit man!
Xylophone Sex: define #2
Xylophone sex
When you take a mallot from a xylophone and shove it up your own ass, or having someone else shove it up your ass, or just going off on the stick.
Dude I totally had some extreme xylophone sex by myself last night.