Well, I must be sorry to say that I don't incline towards you, Mrs. STIDW's view on this.
In your case, all parties feel alright to make this arrangement, maybe only because during the Christmas lunch and long weekend, your ex's new partner needs to join her ex-family or she has some other plan than being with your ex. Then, it could be a good arrangement for all the parties. Nevertheless, your case is anyway quite atypical, and least agreeable to most of the couple in healthy relationship.
Here, apparently, the wife-to-be, the traditional Chinese, is not alright with that, first of all. (By the way, again, how many western women would even be fine with this? It's not about trust here.) Besides, her husband-to-be argues that it's common in western society to travel together even after divorce, which is nonsense. Where does this story come from?
The husband-to-be may not have done anything else with his ex than having fun with his baby, but still, he needs to mind his current relationship as the FIRST priority, namely, to feel for his present-wife-to-be.