This article from the BBC asks the question "Is beer less fattening than wine?" It provides evidence that beer is, in fact, not as fattening as we have been lead to believe -- that it is less fattening than wine and far less fattening than spirits. It claims the beer bellies that abound in the world's pubs are not a result of the beer, but of the beer-drinking lifestyle, which includes a lot of greasy pub food and a lack of physical activity.
Here's a "calories per 100 ml" table they include in the article:
Beer (4.6% alc): 41 calories
Wine (12% alc): 77 calories
Spirits: 250 calories
Milk: 64 calories
Orange juice: 42 calories
Apple juice: 47 calories
By that measure, beer is even less fattening than apple juice.
However, one must consider two things: first, not many people spend three or four nights a week sitting around a bar knocking back two or three litres of apple juice. Secondly, much of the information in the article comes from the British Beer and Pub Association (BBPA), so it is not unbiased.
However, it is an eye-opening read for the moderate drinker. If your sweetie tut-tuts over your one or two "medicinal" beers per evening, you can show her the article and smile -- as long as you stay away from the poutine and the fish & chips while doing so.