Nuts to Butts refers to 2 males riding together on a motorcycle or scooter - often times because the passenger is too lazy to walk to class so he hitches a ride on his buddy's scooter. Guys 2-up on a crotch rocket is the WORST way!
Guy Passenger: "dude I need to save gas money, can you take me on your bike to the store?"
Rider: "nuts to butts? hell no"
The Nuts to Butts line up also has the effect of revealing those recruits who enjoy the experience as they would likely sprout an erection and could be rejected by the commanding officers who wanted to weed out homosexuals.