My partner and I broke up a month ago. I did not have other partners at that moment, my partner did have a fwb. To avoid pronouns confusion: my partner goes with they/them.

I was on a solo trip for two months this summer. Before that time, my partner and their fwb grew closer and closer. My partner and I had rules for our fwb's: it's not a dealbreaker if you catch feelings, but you should tell it the other partner and we agreed to not having actual relationships besides the one we had at that moment. This meant that we would prioritise seeing each other over seeing fwb's in case we would both have busy weeks and limited time to see each other.

I started to get a little suspicious about their bond because I felt like their fwb was catching feelings for them and my partner was not completely honest about their intentions. My partner told me that was not the case and that they had discussed this with their fwb.

However, during that trip, my partner called me to tell me that they've had sex and that their fwb told my partner he loves them. And that they told them they love him back. I was kinda shocked tbh. To me, saying to your partner or your friends that you love them is something different than telling your fwb that. My partner had a different opinion: they thought that 'I love you' in this case was more like an appreciation of the friendship part of their deal. I did not feel right to me.

When I got back from my trip, I learned that they had seen each other a lot more while I was gone. And that when I wanted to plan quality time with my partner, this was getting harder because they wanted to see their fwb as well, while I was starting my thesis.

It all went slowly downhill from there: during my trip I figured out that I feel pressured by my partner to have sex with them. My partner loves sex and wants to do it as often as possible. My libido works differently: it has ups and downs. At some occasions, my partner started to cry when I mentioned I didn't want to have sex. We would argue about the fact that I don't like morning sex. Sometimes I felt like my partner used sex as a coping mechanism for intense emotions.

Finally, we broke up. We figured out our libidos don't match. After our tough convo, we took a walk outside. During that walk, my partner mentioned that they had feelings for their fwb.

I have so many mixed emotions about that. I feel very angry about the fact that my partner has not told me about their feelings for their partner until after our break up. I feel angry that my partner did not take my criticism seriously after they announced their love for their fwb. I feel like their behaviour towards me pressuring into sex was manipulative.

I wonder: is this a case of cheating in an open relationship? And would it be wrong for me to still communicate my anger about this to my ex partner when meeting to exchange stuff?