View from Man Corner. With shields up.
It's always sad when a relationship comes to an end, whatever the reason. I'm probably not in any position to speak, never having experienced the sharp end of marital break-up; on the other hand some friends and family members have and it often saddens me that, having been unhappy together, they don't seem able to move on and be happily separated. People are going to have new lives; it's what happens when you split up, and people are not necessarily going to behave in the most dignified of ways about it.
It does sound as if MW's DH has been a little hasty in getting it together with this person, but I'm sure he has his reasons.
The gym subscription is just a "thing" and I would have thought it would be mild in comparison to the end of a relationship with children. (I'm not quite clear why you couldn't have kept the gym membership, MW. Are you moving out of the area? Or does it have to be a double?)
I suppose, ultimately, he is being a bit of an arse, but maybe if you are both happy, rather than bitter and resentful, it's better for the child to see you that way. Are you happy, MW? Happier than you were when with him, I mean? If you are, then she can then see that, while not ideal, divorce and separation are not the end of the world and that people can still co-operate as parents even if they no longer love one another.
Is he still being a good dad, regardless of goings-on in other areas?
And one more thing - some of you (not on this thread, necessarily, but on mumsnet generally) have "come out" as having been "the other woman" before you began your married lives, and speak of "the ex" in vitriolic tones which posters are invited to join in with, or at least empathise with.