Goingtogetthere, I think it's great that you are friends like this with your ex-T.

But there are a few pieces of information missing here to really have a conversation that doesn't go: "Good for you! You proved us all wrong! :-)" OR "Well something is off about this!"
First of all, we don't know how long it has been since you terminated with your Therapist. Was it a few months ago or a few years ago?
Second, and please forgive me if I got that wrong - you say post-therapy friendships are possible but from what I gather, is this the first time you have seen her in a social situation? It sounds a bit like this when you describe how you talked to her for 20 minutes before you saw the movie and how it was not awkward.
If this was the first time you saw her outside therapy, then how can you call it a friendship? I mean, perhaps I am not getting the whole picture, that's entirely possible. Do you see each other a few times a month? Have you been at her house or she at yours for dinner or for a party? Have your friends met her?
What is that friendship like for you? A distant friend, a good friend, a close friend?
If it was the first time you saw her socially, how can you know that it works long term?

I don't think that anybody really said that post-therapy friendships are impossible. But there are very real ethical problems if we would make it a broad suggestion that "Post-therapy friendship is absolutely fine and possible and there's nothing wrong about it"
Because the problem is not in the friendships that do work out - it's in the hundreds of thousands of friendships that do NOT work out. So you're lucky - you Ex-T likes you enough to be social with you. What about those she doesn't like enough? So you're able to have a balanced friendship - what about those clients who are not able? What about those clients who want the friendship out of entirely different reasons, like simply not wanting to let go? What if a client knew their therapist was friends with another ex-client but isn't very interested in having the same friendship with them?
What if the client has some kind of obsession for the therapist (Which, let's face it, is not very uncommon!) What if the friendship happens and in a moment of need the client friend wants to have the therapist help them, not the friend who might react differently?

And we all know how manipulative people can be sometimes. A little side story.. My therapist friend (never been my T) really liked one of her clients and after they terminated they met up socially. It turned out the client had very obsessive romantic fantasies about my friend and one night disclosed it to her with the desire to pursue an affair. My friend told her that this was not possible and that they had to end their friendship - the ex-client attempted suicide.
How many hurt feelings can result from a post-therapy friendship? So much more than one can imagine.

So yes - there is always an exception to the rule - but these exceptions have to be treated like this - exceptions, not encouraging others that this is something to pursue and giving the impression that it's totally fine.
You make a judgment of sorts when you say:
"What I grapple with is the fact that so many on this board think it's wrong, unethical, damaging"
You know, these things can go horribly wrong, can be incredibly damaging and can go in incredibly unethical ways. And the truth is - in the majority of cases they DO go wrong!
There are only VERY FEW success stories around even in therapist circles, as I have found out from my therapist (who knows almost every T in my town) where a balanced friendship did happen. And these very few balanced friendships happened years after therapy ended. I believe her and I can absolutely see the downfalls of this.

So, without attacking you - because you know I love you a lot for your sweetness - I think it's important to put this topic in context..