There are times when you don't have what you need handy. There are times when it's just too much trouble to go through the whole nine yards - your characters just want a quickie. Sex is not always about somebody getting penetrated.
What other types of sex are there? Well, here are a few:
Slickleggng or Rubbing-Off: Basically the process of rubbing a cock either between the partner's legs or against their hip, thigh, or butt. Feels better with a little lubrication to fuel things. The Catholic girls I knew called it "Nun's sex" since there was no penetration.
Masturbation: You don't have to do a one handed dance by yourself. Men masturbate themselves at the same time, basically watching each other. They jerk off their partner either while they're being stroked or not. Sometimes one partner will take both cocks in his hand and jerk them simultaneously.
Fellatio: Good old blow jobs. You can either give or get or give and get at the same time. Two men can 69 just as easily as a male female pairing.
Docking: Okay, you can't have "sex" this way. However, I've heard of woefully misinformed slash authors writing scenes where one guy inserts his dick into the urethra of another man. For The Love of God NO! This misconception probably comes from the practice of Docking. Docking is where two uncircumcised men press the heads of their cocks together so that the foreskins form almost a seal. It's like a kiss with dicks. Nothing goes in anywhere. And while sounds and penile plugs may be pleasurable (after MUCH training and with someone who is really experienced in inserting them), putting something THAT big up there would rip a cock apart.