Insecurity. Whatever form that may manifest itself in whether it be physical insecurity in how you look, performance insecurity on how well you get down, emotional insecurity in what he may do or think afterwards, etc etc. It could be any one of em or a combo pack of many. The drinking lowers the inhibitions and personal insecurities so you can relax and let the primitive mind be pronounced more without the worry of sober human thoughts and reasoning.
I say try it once sober, but leading into the sex with an "I don't give a fuck" pre-loaded mentality. View him as there to pleasure you and KNOW he thinks you're the sexiest bit o woman he's been with. You have NO IDEA how sexy that makes things when the confidence in the air is so palpable you can smell it. When you're confident, you're comfortable. And when my women are confident and comfortable, i feel it and in turn it gets me more into the sex and you both end up feeding off each other into truly memorable sessions.
Let go woman. BE sexy and you won't need the booze to be "sexy" for you.