The idea that moderate consumption of alcohol is "perfectly healthy" is actually quite debatable. Taking red wine as a an example because of the popular belief that it has notable health benefits when consumed in moderation, a few points:
1. Many of the studies on the supposed health benefits of red wine are inconclusive. For instance, we still can't explain the French Paradox.
2. Last year, it came to light that a researcher who was behind some of the studies on the benefits of resveratrol falsified data. A decent amount of the hype around red wine in recent years was centered on resveratrol and it's important to note that even if there are benefits, the actual relationship between compounds like this and human health are in most cases not yet clearly established.
3. Red wine contains known carcinogens.
4. Even moderate consumption can add significant calories to one's diet. For example, if you're a male and drink two glasses (which is considered a moderate, "healthy" amount) three days a week, you're adding nearly 3,000 calories to your monthly caloric intake. Depending on when and how you drink your wine, you might also consume more food because of wine's impact on hormones associated with appetite. By the way, this is even worse in most cases for beer, where with some major brands, a single can or bottle a day can easily add 4,000+ calories over the course of a month.
5. Many of the potentially beneficial compounds in wine are available through other food sources that come with fewer drawbacks. In other words, you don't need to drink a glass or two of red wine every day to get your antioxidants.
The bottom line: if you're going to enjoy a glass or two of wine (or beer) with a meal, you shouldn't justify it on the belief that it's "perfectly healthy".