My wife was pregnant
When I first started dating my wife she was pregnant. My wife had just broken up with her boyfriend after being together for almost two years. She wasn't showing yet but after a month of us being together it was getting noticeable particularly when she was naked and she was having morning sickness. I wasn't concerned because I could see that we were good together and I thought that I really wanted her to be my wife.
My wife went to the doctor and he confirmed that she was indeed pregnant. But he also told her that she was around three months. Since we had been together for only a month, it was obvious that the baby was her long term boyfriend's. When she told me she said that I could leave her and she would try to get back together with her long term boyfriend. I told her that I loved her and wanted her to stay with me.
We got married a few months later and she had a girl shortly after. We have never told anyone particularly our families that our oldest daughter was conceived by my wife with another guy. She at twenty is the spitting image of my wife when we first started dating so everyone doesn't have a hint that I am not her biological father. Our two younger children look more like me.