The Dangers of Settling (Spoilers Galore)
The problem with the whole marriage stems from the fact that Jo "settled" for Michael because her relationship with Michael did not work out. Since she was still pining for another man, she never gave her husband the emotional connection he kept trying to have with her. That led him to fall easily when tempted by a strong sexual attraction to someone else.
You should only marry someone if you heart want that person alone. Anything else is cheating yourself and your spouse and creating later problems