What causes married woman to cheat on their husbands?

I am a single male (19yrs old) who nearly committed adultery with an married older woman (36 yrs) with two kids. We arranged a ual encounter at a distant hotel. Prior to us trying to get ually intimate, she showed me pictures of her family (husband included) and I felt very bad deep down inside mainly because I grew up in a loving family and had a very happy childhood. I told the married woman I can't go through with this and she got desperate and tried to hold me down physically. She keep saying 'please stay, make love to me just once and then you can leave' and I was quite appalled by her desperation. I am wondering why married women cheat?