AITJ for telling my boyfriend the agreement his made with his baby mom was not feasible while married?

I 24F have been with my 23M boyfriend for a little over a year. He has a three year old boy and I have a 4 year old boy. My son’s dad was killed in a motorcycle accident when he was four months ago and my boyfriend and his son’s mom broke up shortly after their son was born

About six months into our relationship, he decided to tell me that he and his son’s mom agreed to split up because their relationship wasn’t working and that they would still do things with him as a family even when married to other people

I told my boyfriend that I wish he would have told me that in the beginning so that I could have made an informed decision on whether or not to pursue anything with him because that agreement was not something I was comfortable with. The reason why I said this was because the entire time we were together I was under the impression that he was with his son and spending time with him alone and that she would join on occasion, but I was made aware that she was coming along pretty much all the time and I was unable to spend time with him and his son because she was there and did not want me there

I told him that he should’ve let me know that in the beginning and he agreed and apologized. I then explained to him that the agreement likely worked when they made it and until he got into a serious relationship with me, but I do not want to marry a man and have kids with him when his free time is spent with his son AND his baby mom. I always had in my head that co parenting was two people taking care of the child from separate homes, spending time together during birthdays, school and sporting events, and other important life events. That child then being included in both families lives once the child’s parents remarried and had other children. My boyfriend then told me he likely knew this would be a conversation in the future and he ended up talking with her and it didn’t go well

She recently got into a relationship and her boyfriend was made aware of this agreement as well because he apparently asked about it. My boyfriend told me he told her the same thing I told my boyfriend. My boyfriend agreed that it was an agreement two people who had never been in previous relationships made between one another and that he kind of figured that it wouldn’t last once he remarried and when she did as well. Out of all four of us, she is the only one who is upset that the agreement is ending. It got to the point where my boyfriend was on the phone with her and her boyfriend was in the background asking her why she was so upset almost as if he was accusing her of holding out hope that they’d get back together

She ended up tearing up and her voice got shaky and she hung up. He tried to call her back and she didn’t answer. She then sent him a message saying she felt like I was being chosen over their son and my boyfriend told her that I (person writing this post) did not want him to stop seeing his son and told them that I actually wanted him to come on our upcoming vacation to Disney bc my boyfriend wanted to go see the Harry Potter part or something bc he’s a big Harry Potter fan and it was supposed to be a birthday gift to him and a family vacation and I bought us all matching Disney shirts. I bought one for his son as well and told him to ask his son’s mom if he could come with us

I asked him if he had ever set boundaries with her and he said that the entire reason why they have issues co parenting is because of her lack of respect for boundaries that have gotten worse. He told her about a month ago that he was taking her to court for a more uniform agreement and she told him good luck whatever that means