I have really struggled with this - not that this is happening or I see it coming. But, what do you talk about with this person now?
I have had the exact same problem and we still live under the same roof. At home you can try being in another room and busy yourself but with a kid's activity you both need to be engaged and it just became way too uncomfortable for me. Maybe when detachment is complete it will feel less awkward but it will always be a farce I think, doing it just for the sake of the kids because they want the parents involved. You have a good point Nicole, either they are 100% involved or not do it at all, half a$$ing shouldn't be allowed, being present both mentally and physically at the activity should be a pre-requisite. So until the day that estranged spouses can be ok in each other's presence I guess we fake it for the sake of the kids.