Marijuana contains delta-9-tetrahydrocannabinol, more commonly known as THC. THC gives the illusion of altered senses, like brighter colors, or the sense of time.

What you're experiencing is an illusion of senses brought on by THC, thus perciving that the act of sex is better. However, like Webgoji said, continued usage actually causes ED.

Please be careful about how you use it.