Tupac's Sex Scene Drawings, Love Letters From Jail Hitting Auction Block

Tupac had sex on the brain when he was locked up, because his rare hand-drawn love letter to his girlfriend is hitting the auction block ... and the images are extra erotic!!!

Pac was probably humming "How Do U Want It" when he was drafting up this card from jail, because the drawings show him and his girl, Desiree Smith, GETTING IT ON!

Steiner Auctions says the rare hand-drawings, envelope and card were all created by Tupac in 1995, when he was incarcerated at Clinton Correctional Facility in Dannemora, NY.

Seems Tupac was already a big fan of his future Death Row labelmate, Snoop Dogg ... because Pac is definitely down with doggy style!

Of course, it's not all about sex. Tupac gets kinda mushy with his words, telling Desiree he loves her and misses her deeply (pun surely intended).