She's not cake eating in that respect. She wanted to see her/your son. There will be times when it will be her time with your son and you will want to be with him also. You've got to learn to compromise.
Maybe I didn't post the complete deal here, but what I ended up agreeing to was we went to church as a family...she got S from after church up til 530pm. I softened at 530pm and asked her if she wanted to stay for presents and cake. She'd already done this once with him, but she stayed. So while she wanted us all to be together, she didn't get that, but I do think I was more than fair.
With that said, you are right...I was angry and hurt and I wanted to have her feel the results of her decision as quickly as possible, which meant we split the day rather than shared it. I honestly didn't think I could be around her that day, but I could have bowed out of the activities a little more graciously. Distinct boundaries could have waited one more day.
I feel like I have been carrying the load for the family for a long time now, and sometimes I just want to put it down. Some days I am just so worn out, it's difficult to do all the "right" things. No excuses, I did not behave how I want to be. I'm really getting good at being who I want when things are calm....the high stress moments are still a work in progress.
I'm going to try to get myself in a mental state of "we're just friends taking care of kids together" and see how I feel about that. I want to be upbeat, compromising, understanding, but not a doormat.