All my clients on the initial stages of my programme aren't allowed to drink (amongst other things). The reason is that I don't want anything to hinder the effects of the system. I hear people say, "what about a gin with slimline tonic? It has hardly any calories." Calories in alcohol aren't necessarily the main problem. You actually don't store much alcohol as fat (so long as you haven't consumed excess calories for the day). I can see you reaching for the bottle as I type. The problem with alcohol is your liver converts it into Acetate. Acetate is then released into your bloodstream and stops fat burning. No protein, fat or carbs get a look in when you have Acetate in your bloodstream. Acetate also contributes to your hangover.
So if you don't put on weight from alcohol alone, where does the weight gain come from? The weight comes from the excess calories consumed after your session. This could be from alcohol or food. While Acetate is in your bloodstream, your body will convert excess calories into fat. I refer to alcohol as 'all the calories and non of the benefits.' You can't use the calories in alcohol to exercise the same way you would with a chicken salad. It would be like saying 2 bottles of wine will get you through a triathlon. However, the excess calories from your session count to your 'allowance' for the day. So whatever you eat after pushes you over. You can also get water retention which can add to the weight gain.
So it is far easier to knock it on the head for a short while to accelerate your fat loss plan.