Thanks all for the feedback. A couple of points and answers to questions.
Grubs asked if we sit together if my GF is present. Yes, absolutely have in the past. Rewind a year for another school marching band half-time show. Me, my youngest (my weekend), my GF and her son, the ex, my former MIL before she got sick, the guy my ex just dumped all sitting (proximately) together for the half time show my oldest and the exBF's son were doing. Not the only instance. Not always, not always that full lineup, but yes, it has happened.
We do this for multiple reasons. Available space. When her mom was present, which was often, I remain friendly and enjoyed seeing her. The woman helped get me into law school after all. But mostly to show our kids that they come first for us both. And we don't need to be openly hostile to one another. They were 9 and 7 when we split and they were exposed to some ugly stuff from us both early on. The older one comes with similar anxiety and other emotional challenges that his mother deals with. Once we got into and through the divorce process (our second from each other, btw), some of the anger subsided, we both agreed to not let it splash on the kids again. Further example, I think second holidays after we split in 2017, she had the kids as scheduled and my mom was visiting. So she hosted me, my mom, her mom for dinner because that was the narrow window available for my mom to see her grandkids. Because that interaction with my mom is important to the boys and me because my mom's would have been 82 at the time and there aren't going to be many more chances. This flexibility applies when either my brothers or her sister are in town visiting as well. Family beats schedule.
The ex has been single for a couple months, but the idea we would end up either "back" together as you suggest or even a one-time fling is simply not possible. Navigating for minimal conflict is just maintaining my indifference.
To RocketRacoon, as indicated above, not the first instance. Mostly situationally driven and my GF sometimes (schedules, etc.) also there. As I indicated, haven't posted in several years. Much of those previous posts served as vent and support and "misery loves company" mind set at the time. Certainly not in that place anymore.
To KitchenDepth I will say she didn't initially get emotional about the relationship. It started with the seat for her mom and "she should be here, damnit" that kicked it off. I was feeling down about it as well because her mom and the 8th grader we were there for were close and I felt bad for what he missed. She just tossed in the news about the exBF. And my response literally was a pat on the back and either what you said or "that's too bad." But the grief we both felt for her mom was real.
And for the latest kids half-time show, we basically sat there and chatted about innocuous stuff like adults. So when I say "If I can't sit there" like a couple of adults when the kids are involved (and the place was insane and I hate crowds), it's driven by all this, by two marriages to each other, two kids, an abortion, an affair and three decades (on and off) of history and baggage between the two of us. Maybe at my age, I'm just looking for a little peace in my life?
So the "things" my GF may be considering aren't really in me. Which we've discussed. And the "miss you" texts and whatnot are not happening, have not happened shit even when we WERE married, and will not happen. My ex's affair for her was an exit affair. I got knifed in the back, but whatever. I also haven't hidden anything. I could have. But I didn't because it doesn't matter to me.
And if doesn't matter to me, how and why does it matter to anyone else? Do I not get to decide how I navigate my co-parenting relationship with my ex?