Guys guys guys... you are all missing the point. He can't just go into the bathroom to shoot. He needs a cover story.
Follow these steps:
1. Walk around the apartment for awhile loudly announcing, "The Wood machine is up!"
2. Start collecting porno mags. Ask your roommate if he has any more hidden away.
3. Pick up a jar of hand cream and spray a glob in one hand.
4. Grab a couple paper towels and shove them into your back pocket.
5. Head into the bathrrom announcing, "Gonna get me some love tonight!"
6. Once inside the bathroom wash the glob off your hand, relax, and shoot.
Take as much time as you need. Your roommates will not come checking on you.
You can thank me for this plan by sending karma.