I think there is a lot going on here.
First, it's not wrong, abnormal, etc to fantasize, what is wrong is to have jealousy over it. Now if one was to act upon those fantasies without a mutual understanding between the partners of what is an agreeable relationship, that is another thing (I personally do not think it's wrong to have a mutually acceptable open or a form of an open relationship).
Second, we are getting one side of the story, others have pointed out, the mention of fantasizing could have been freely given or pulled out of the spouse, the latter probably a result of jealousy.
Third, I'll say it again, love is not sex and sex is not love. They are shared, but they are not mutually exclusive terms. One can be attracted to one person, but be in love with a different person who they are attracted to physically, but less so than the other. There is more to love than physical attributes.
Fourth, I do not agree with the husband stating that he thought the bridesmaid was more attractive on a wedding day than the spouse. But again, was this said in anger, frustration (not that it makes it better) or freely given. Sometimes people say stuff in anger/frustration, that has truth to it, but would never say normally. It doesn't make it right and the person needs some counseling to handle their anger. Now if it was said freely, well, that person needs to learn when and how to say things, the same thing could have been said with a positive influence and not have been as hurtful.
All in all, I think some open dialog is in order, not yelling or screaming, but honest, open dialog. This probably is best with a counselor present who can be the mediator. It also means separate counseling sessions with each person to level set and keep the communication flowing.