It may be weird to phrase it this way, but cheating has been a real part of my relationship with my husband. I have cheated on him several times. As far as I know, he is only aware of two of the times, but there have been many others. He has cheated on me as well.
If he is telling the truth, his cheating did not involve sex. He met someone online and was buying her gifts and sending her money. He claims his fetish was all about the gifts and money and he never had any interest in anything physical. I believe him because he is not overly into physical sexual relationship and I know he has a kink about giving money.
My cheating has been physical. I'll start with the two times he knows about:
Before we were married I was had a female lover on the side. When we decided to get married, I ended the relationship and confessed to my husband. He wasn't happy, but was very willing to forgive. I suspect it was because he really was turned on by the idea of me being with a woman.
A few years after we got married, my husband started taking some medication that completely killed his sex drive. After months of no sex I was getting desperate and in a moment of weakness I had sex with a male friend of mine. The sex was so great that we continued it for a long time. Six months into it, my husband approached me and said he knew I was having sex with my friend and he wanted me to stop. I told him I would only stop once he and I started having sex again. I continued having sex with my friend for months, and made no attempt to hide it. It was an interesting period in my marriage with me openly cheating and him just letting it happen. Eventually the medication was no longer needed and I stopped cheating.
Now the ones he doesn't know about:
I have a cousin who is a lesbian and I have had sex with her wife. The first time was at Christmas in a spare bedroom at my parents' house with family everywhere. Since we got away with it, we did it a second time the following Christmas.
A few years ago our neighbor used to have Lost viewing parties and I would go over her house to watch the show. My husband wasn't into the show and he never joined us. One of the guys that attended the party would leave his wife at home for the same reason. We started having sex in his car after the show, and when the season ended we kept meeting up once a week for sex. We stopped when his wife found out and he got divorced.
I don't go on a lot of work trips, but about 18 months ago I traveled for my company and had drunken sex with one of my coworkers. I thought it would be awkward to go back to work with him, but we've never mentioned it to each other.
I've recently started having sex with a woman who is in my transgender discussion group. The sex has been great and I don't plan on stopping any time soon.

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