I'm a physician and I can tell you that addiction is a disease. The human body can become physically dependent on chemicals such as alcohol. In fact, in folks that are alcohol dependent, withdrawal can be very deadly. When we have alcoholics in the hospital, we often have to give them medicines called benzodiazepines to prevent withdrawal. The pharmacy at many hospitals will also carry beer to avoid withdrawal, if for some reason benzo's can't be given. You will commonly see alcoholism running in families, which possibly suggests a genetic component making some individuals more susceptible to physical dependence. There are also fellowships that train doctors to specialize in addiction medicine. To summarize: Addiction can cause morbidity and mortality and oftentimes patients need medical help from addiction specialists.
Alcoholism is definitely a disease. There are many who are more prone to becoming addicted, and once addicted it is a very physical thing. Both the mental and physical dependence is a disease that needs to be treated properly if the person intends to get better and overcome the addiction.
There are some people who can drink every day and never become an alcoholic and then there are people who are an alcoholic after they have their first drink. There has been shown to be differences in the brains of addicts that normal people do not have. There has also been shown to be a strong genetic and family connection.
There is no addiction to being homosexual, it is simply that you was born that way and you are not harming yourself from it. Alcoholism however is developed during a persons lifetime and affects them physically and mentally. This gives it the attributes of a disease. In order to recover from alcoholism, treatment is needed as well as support and guidance. This again makes it similar to that of any other disease.
I'm no biologists but I learned in school that the rule of thumb is that diseases are caused by bacteria and infections by viruses. Considering alcohol isn't a bacteria I don't think it causes any kind of disease. It's an addiction like video games (which I'm addicted to), tobacco, drugs, and etc.
Having been married to an alcoholic for three years I've seen the problem destroy a man and take his life. I do not however believe it is a disease but I do believe there are genes that can make a person more prone to addictions. Homosexuality and alcholism aren't comparable and I don't know why anyone would compare the two.