FMF or MFM
My wife and I had been together for two years and married for five when I asked her if she was feeling the seven year itch. She said that she had been thinking about how she had liked the variety of guys that she had fucked in college. I told her that I had also enjoyed the variety of girls that I dated. I asked her if she wanted to swap partners with any of the couples that we knew. She said that she didn't want to take the chance of messing up our friendships with them. I asked her about a threesome. She didn't want to share me with another woman, she liked having me all to herself in bed. I told her that I was thinking about having another man join us so that she could be fucked twice as many times. She asked if I would really let her have sex with another guy without me getting to have sex with another girl. I told her that I just wanted her to be happy and I thought adding another man would be fun for her.
We did this for nearly two years and my wife had sex with over 50 men. I never felt left out because I always got to have sex with her after they finished. Fucking her after another man had come in her felt fantastic.
Then we decided to have children and she stopped having sex with others until we knew that she was pregnant by me. We found that men loved fucking her while her belly was getting big. After she had the baby we stopped until she got pregnant by me a second time. When we were ready for a third child I told her that I wanted her to have sex with other men and have one of them get her pregnant. So our third and fourth children may have been fathered by me but may also have been fathered by a couple of the guys that my wife was also fucking. It didn't matter to me whether I was the father or not, they were our children even thought someone else may have fathered them.