In defense of those who subscribe to the idea that alcohol is a sin itself, many times it stems from a relative or themselves having once been affected by alcoholism. Over-eating is a sin because it dismisses temperance. The repercussions of over-eating can also lead to being slothful and idle.
For many people, drinking alcohol can many times mean over-drinking, just like eating, so temperance is also dismissed there too--even if drunkenness never happens. Although, when it does happen, you don't just have slothfulness as a repercussion, you also have impaired judgment which can lead to even more sins.
Of course, I wouldn't argue that drinking alcohol is a sin. Personally, I don't care much for it and don't drink it (I'm very childish when I order drinks, I will order Shirley Temples). However, like anything, it can be abused. Sex is a wonderful thing that is very often abused, even within marriages. Eating, which is a necessary activity for everyone, can be abused. Drinking alcohol is often looked at as a sin because abusing it, like sex, isn't difficult if one wanted to. Of course that doesn't mean that it is a sin when it is not being abused and treated with responsibility.