Todays society tells you its ok to express your feelings no matter what. There are no boundries for some people. They let it all hang out at work, church, public places and in situations previously thought inappropriate for that kind of behavior. My H felt our marriage was over and led the OW to believe that divorce papers were minutes away. However, she knew he was still married and that he hadn't talked to me AT ALL so go figure.
Love always protects, always trusts, always hopes, always perseveres.
However, I think a few things can be said in addition. In many cases, I'm sure, the spouse instigates the relationship. In my case, the OW made it clear to my H that she was attracted to him, and was willing to take to the next step, if he was. The OW was a chronic flirt, flirted with everyone. I really think that this type of person is very insecure and has very low self-esteem.