My therapist asked me if I would stay with someone who was cheating on me. I said no. He then told me that staying with an alcoholic is essentially the same as staying with someone who is having an affair. It is their obsession, it is their passion, and it takes them outside of the relationship. It hurts the relationship, and they do it anyway. In fact, on those terms, it is a little worse. It's staying with someone who is having an affair right in front of you.
This made sense to me. I don't know if it's coming second or not, but it sucks.