I did something similar in my first de facto relationship. The guy concerned was a friend of his. At first my partner enjoyed knowing I was having fun with his best mate but eventually he told me he didn't appreciate it. I also came to realize it was no longer fun anymore so was happy when the other guy got married so I used that as an excuse to graciously bow out of the situation.

We were silly enough to do it all again later with someone else, and this ended with my ex leaving when he realized I was falling for this one. He could handle me having fun sexually but that was all. I ended up staying with the next guy.

Now looking back, I've come to realize that neither my ex nor myself had much self respect at the time. I'd hurt him for a bit of fun and ended up feeling cheap as I knew the first guy was just using me for easy sex. The second relationship was rocky. Even though I stuck it out for several years, it never had a strong foundation.

I've done a lot of growing up since then, and would never put myself nor my current boyfriend through something like that. I just take advantage of my fantasies and Pixies to get my extra thrills nowadays.