1) A penis, or phallic object.
2) A dildo pen. Thus a pen you have sex with.
3) A play pen for orgies.
1) "Can I play with your sexpen?"
2) "Dude, I forgot to take my sexpen out of my bag, and it almost fell out in front of my boss!"
3) "Would you like to join me in the sexpen, its an all-nighter."