I don't know the exact dynamics of your relationship with Big M, but I do things with my DD's father and DD (so, just the three of us) relatively often because DD enjoys it - things like going trick or treating or out for pizza after a school conference or to Disney Princesses on Ice. I am in a serious relationship. I suppose there are unresolved issues between her father and me, but we do these things for no reason other than it makes our DD happy and that way she doesn't have to choose which parent to experience something with. My SO, who also has children, encourages these outings because he understands it is not about me and my ex - it is for the joy of our daughter, who my SO cares about also.
I am not saying the OP should go to the game (and I know that's not an option anyway) but I would NOT assume Big M wanted to go to the game as some sort of date with the OP. It could very well be he wanted to enjoy something as parents of DS. It's not the same thing.
Edited because I forgot the word NOT which totally changes the point of my post!