The social lubricant that alcohol can be at times can also be a huge downer when it comes to the bedroom. While alcohol may lower some inhibitions and heighten feelings of sexual arousal, it has the "bang your head against the wall" affect of impairing sexual function. Fun, huh? Didn't think so.
So what exactly is impaired at the time? For men, they have a difficult time staying erect and may not reach orgasm. For women, natural lubrication is blocked, sensitivity is dulled, and orgasms are inhibited.
Beyond the climax issues, there are also the basics of the "get down" which can be affected. Due to the lowered fine motor skills and issues with balance, two inebriated partners may have a tough time maneuvering together, staying on the surface they are using to support themselves, and other issues which can lead to unintentional and possibly embarrassing injury. And a sex injury just may not be worth it...
Having sex while under the influence is not guaranteed to be a bad time. Alcohol just adds more obstacles to the equation, including a reduced ability to use contraception properly, a lack of understanding communication clearly, and the issues above. If you choose to drink and would consider engaging in sexual activity, consider what is best for you and your partner.