When I tell him that divorced couples celebrating all holidays and special occasions together is unrealistic, he generally says that I can’t expect him to stick around forever if we can’t get along well enough to spend time together. I’d assume that the marital issues would get better with time while the divorce would make things worse between us.
This seems like something simple to explain. Married people trying to make their marriage work want to spend as much time together as they can. Divorced people, who are hurting and trying to heal, do not want to spend time together as this is emotionally painful and slows or even stops the healing and moving on process.
Divorced people can certainly get along and be civil or even friendly when it comes to raising their children and discussing the children, but they tend to want their ex out of their personal lives so that they can heal from the pain, get used to being single, and eventually date and remarry.
I think being friendly and civil is what the therapist may have been talking about when she said her bit about “co-parenting”. IE: You talk about the kids…rules, who got grounded and shouldn’t be allowed to go play on visitation, who got sick, how their grades are, etc. But no more than that. Certainly not going out and doing things as a family or spending holidays as a family because divorce means you aren’t a family anymore. You’re two adults who treat each other civilly and with respect, raise your children as more or less business partners, and stay the heck out of each others lives because you no longer share those lives. And that’s a best case scenario.
As we’ve said before there is also the possibility of new love interests to consider. It’s totally unrealistic to expect new husbands and wives to get along with the exes well enough to be willing to spend holidays together. They’ll have their own families and traditions. They’ll likely feel threatened by a too close relationship between exes. If he were to find a new woman I am pretty sure she isn’t going to want to be your BFF, too.

Nor would any future love interest you have want to be his BFF. And, frankly, once you’ve moved on to new relationships your loyalty would have to be with your new partners.
I think both you and the therapist need to be more clear on the realities of divorce. I’m all for plain speaking. I’d tell them both that just because divorced people could be BFF’s doesn’t mean
you’d be willing to live like that. Explain that you’d be healing and trying to move on which wouldn’t be possible with him so heavily involved in your life. So, it’d have to be the standard divorce. No coming to the house without good reason and prior phone call and permission. No shared holidays. No going out as a family. NO CAKE AND EATING IT, TOO!
I know Xantippe says not to mention it now, but I would. I’d want him to totally understand what he’d be doing** before** he filed any papers or talked to any lawyers. Often once a person has gotten to that point they have already made up their minds. Wouldn’t want him making up his mind and taking action believing things will be much different they they would be.
As a 10 year veteran of marriage I do believe things could greatly improve between you two and you could have a great marriage. But you can’t make that great marriage alone. You both have to believe things will improve and work from that assumption. First assume that it will get better and then find ways to make it better. When we were having marital troubles my husband flat out told me that and he was right. Once we were both believing and acting like the marriage was going to get better, it did.