Alcohol tends to have a negative effect on male sexual performance, since alcohol intoxication can decrease sexual arousal, decrease pleasureability and intensity of orgasm, and increase difficulty in attaining an erection, also known as whiskey dick. However, men that suffer from premature ejaculation may actually benefit from the decreased stimulation.
The effects of marijuana on libido are inconclusive, but marijuana does not appear to have negative effects on sexual performance. Studies have shown that marijuana boosts libidos, increases sensitivity to touch, and enhances erotic pleasure in roughly 50% of people who smoke. (13)
13. Osborne GB and C Fogel. "Understanding the Motivations for Recreational Marijuana Use Among Canadians," Substance Use and Misuse (2008) 43:539