If you were married and started to have strong feelings for/be smitten with someone else, would you stay or go?

Your description of the marriage is "nice" and "decent." Therefore, I surmise that it's an uneventful marriage. The result is the brewing of the need for compion, communication and desire, which someone is exhibiting to one the spouses. Don't bite the bait. Instead, the married couple should talk about how they treated each other while dating and first married and work together from there.