Here are the facts. At first, marijuana actually increases the amount of blood going to the brain, and everything seems extremely interesting and exciting, or terribly funny. But about 45 minutes later, cerebral blood flow actually dips below its normal conditions. The problem is, you can't get something for nothing in nature. Your judgment is compromised.
Alcohol works a little differently.
It hits the central nervous system, acting as a depressant, so it has a sedative effect. Some of you have probably heard about kids who drank too much and passed out, who didn't remember where they were, or what they had been doing. But anxiety is an important signal in the decision making process. With all these considerations in mind, imagine if marijuana or alcohol was thrown into the picture. Decisions about sex cannot be made casually.