My long distance boyfriend wants to remain friends with his ex.

My long distance boyfriend wants to remain friends with his ex.

My boyfriend (29m) and I (24f) have been in a long distance relationship for roughly 3 months, and he wants to remain friends with a toxic ex. Being friends with this certain individual is a hard boundary for me and I told him he needed to choose either his relationship with her, or with me. He asked for time to “think” about it and contact outside sources such as his close friends for him to decide what to do. Am I in the wrong for placing this ultimatum on him? I feel that it’s ridiculous that he even needs to think about this decision, but he really values this friendship with her. He says that he has no interest in getting back into a relationship with her because they brought out the worst in each other, and they were very miserable, but he holds on because they were great friends and he knows she’s someone he can lean on during tough time and vise versa.

For some context: We met earlier this year and fell hard, fast. In the beginning, I thought of myself as the type of person who could be okay with their partner being friends with an ex. I deeply value friendships and I would hate to be “that girl” that makes her boyfriend stop talking to certain individuals. So when the topic of being friends with an ex came up, I said I would be okay with it. But, as he told me more and more about his ex it became clear to me that HE was the one to break things off, leaving her very much in love with him and hoping for another chance in the future when they’ve had time to “work on themselves”. When I figured this out I asked him to stop speaking to her. He talked to his friends for some advice and then told me he has asked her for some space to allow our relationship to develop without any issues. I believe this to be entirely true. Since then however, this has always been a touchy subject for us.
A couple of weeks ago I had asked him out of sheer curiosity if he had spoken to her since. He said no. When he came to visit me a couple of days ago we stumbled across a hidden DM from his ex. He immediately denied any knowledge of it and explicitly stated that he hadn’t had contact with her since he asked her for some space. Come to find out, this was a lie and he had been speaking to her now and then since the couple of weeks ago when I asked him if he had. I’m torn, I feel betrayed and stupid I hate that he lied and she is causing a hiccup in our relationship. What would you do? Work things out or break it off?

TL;DR my long distance boyfriend of 3 months wants to remain friends with a toxic ex and I told him he needed to choose either his relationship with her, or with me. Am I in the wrong for placing this ultimatum?