I certainly wouldn't ask your XH to decide between you and ow. He already made that choice once, sorry.
I'm afraid I think yabu. I thought you were going to say it was a recent new relationship, but after 2 years they are an established couple and there's no reason why she can't be a part of your DS's life.
I met DP a year and a bit ago. Last year my DS asked if DP would come with me to watch his music concert. I did mention to XH that it was DS's idea and he was fine with it.
I've also been to watch DP's daughter in a dance thing. His ex saved us all seats on the front row.
We all spend lots of time together despite not living together, so the fact that your x and ow don't live together doesn't mean he doesn't or shouldn't see her regularly. As his dad's partner it's quite likely that she is around a lot and if she cares about your DS enough to stand out in the cold watching football, you really should be happy about it. No, it doesn't detract from what they both did to your marriage and family, but you need to let that go for the sake of your DS.
I appreciate that it helps when the relationship happened after the split, but whatever the circumstances, you need to accept that this is your ex's life now and the people he spends it with are entirely his choice.
Please don't make things awkward for your DS by causing a scene or making a point. Sadly any sympathy you might get from friends for being wronged by the pair of them will quickly disappear if your upset your DS.