I realized several years ago that I got into it as a teenager. Most of my girlfriends cheated, but two really set me up for it.
The first was "Sally". She cheated multiple times. The first time we broke up, but got back together a few days later. The second time we fought bad, but made up by the end of the day. The third time we argued, but not bad. After that she just told me she had cheated and I got upset, but let it go. She was wild and I knew it. I should have married her.
The second was "Donna". She told me from the beginning that if she was going to be my girlfriend I needed to accept she was going to fuck other guys. Word was out that I had a little dick and she was just being honest with me. I agreed to it.
So when I was 23 and my 19-year-old wife expressed sexual unhappiness I knew what needed to happen.