Possibly. The same type of chemical makeup in their body that causes them to have trouble with alcohol addiction and impulse control could also affect their fidelity in marriage. Another thing to consider is that those who imbibe often and become drunk are more likely to find themselves cheating.
No. Affairs are not tied to those who take alcohol. Alcoholics are not so much likely to have affairs although this is much determined by the reason that make one an alcoholic. If the alcoholism is due to family or domestic misunderstanding, the likelihood of and affair can be suspected.
Alcoholism can impair someone's judgement so that they might do something they normally wouldn't. However, having affairs is more about not respecting your partner, and you don't have to be an alcoholic to be that way.