You know him best, or at least you thought you did. At least he thinks enough of you to come clean and admit it. It doesn't necessarily mean your relationship is over. Some men do stray. Many decades ago when I was in my teens my employer came on to me. What could I do but agree, thinking I need this job. After a time I wizened up, decided I would move on from the job and wrote an apologizing letter to his wife. He must have pulled his head in as to this day they are still married and retired and happy. Sometimes it just happens. <3