Well not a spouse but a girlfriend. I have to start off by saying before we dated, we were good friends. Anyways, we had a fight one night and she drove off from my house angry and went to go blow off steam at her ex boyfriends house and cheated on me with her him but initially didn't tell me, she just ended the relationship a few days after. Then a few weeks later she showed up to my house crying and admitted to what she did and told me why she did it. Naively, I took her back because she made it very clear she only wanted to be with me and promised it would never happen again. Well that was bullshit. The next few months were fine but she dumped me again. I suspected it was because of the ex boyfriend. About a month after our break up she came back to me. Naively, I took her back. This whole break up-get back together process happened like 3 more times over the course of a year. But sometime within that year, during one of our break ups, another, much nicer girl, I was friends with admitted she had feelings for me so I started seeing her also. Unfortunately, I kept getting back together with the first girl because I was so infatuated with her. By the end of the relationship I was tired of her shit so I dumped her and then she got back together with her ex. Then I went to go be with the other girl but the other girl didn't want anything to do with me. Then a couple years later the ex texted me saying how she missed the friendship we had before we dated and wanted to know if I wanted to come over to her place for dinner. I said yes and that's when I met her roommate who was also her best friend. Her roommate was pretty attractive so I used our new "friendship" to get in close to her roommate, the three of us hung out a lot. Anyways one day her roommate came over to my place and I made her dinner, we had some wine, then we watched a movie and things got a little heated between us. Naturally, we did the deed. That put a strain on the ex's relationship with her best friend. Then months later I ran into the ex's older sister at a park and we get to talking and we made plans to go get dinner together and catch up. After dinner we go back to her place and hang out for a little bit and she invites me to stay the night so I did. Yes, we banged. The ex found out I slept with her sister also and from what I understand, she was not happy about that. It's kinda petty but I'm glad I got to dish out some pain to her since she caused me a lot of hurt when we dated.

TL;DR: girl cheated on me, I revenge fucked her best friend and her sister.