It depends upon the quantity of alcohol you consume. An amount of 30-50 ml of drink (the usual peg) acts as a good stimulant while larger quantities result in poor performance. There is a saying that alcohol provokes the desire but takes away the performance (in large quantities). If you are able to have normal sex without alcohol, there is nothing to worry about. However, if there is a problem in getting good erection in non-alcoholic performance, please seek treatment for the same.