you can not use very specific situations to prove this point. Mostly if the other parents doesn’t want them to meet the new partner they shouldn’t
obviously we both have our views.
But for how long is the parent supposed to bow to the ex's demands?
why should the parent with the new partner not include their girlfriend or boyfriend because the other parent wants to be spiteful and probably jealous. One parent should not have to put their life on hold because the ex doesnt like it? not inviting new partner to family bbqs because the kids are there, not taking them to a family wedding because their ex doesnt agree with it, what about when they move in together? tbh i dont know anyone that would put up with that either professionally or socially
if anything as a parent they should be glad that there is another person in ther childrens lives to love, protect and nerture them and trust that the other parent chose someone suitable. eventually both parents may move on to a new partner.
After seeing their parents upset, the children may well feel better and more relaxed seeing their parent happy rather than unhappy due to their ex controlling them by refusing to let new partners be introduced.
I agree to tell the parent first out of respect that an introduction will happen but thats it.