"I probably am a bit paranoid, but 18 months ago I had a shock when he told me he had been seeing an ex girlfriend and had to decide which one of us he wanted to settle down with. He kept the liaisons with her very secret too...only telling me post-event when he had to make his mind up because the pressure of the situation had started getting to him."
That should have been when you told him, "I'm going to make this very easy for you", and then said that he's welcome to the ex and ran for the bloody hills.
But hindsight is a wonderful thing... So, basically, here's what we have:
On the one hand, a man who lies, conceals, hides the truth, actively prevents you from being part of his daily life (FB)..
And on the other hand, takes HUGE umbrage when you don't just roll over and accept this, and makes out you're somehow deficient by not happily accepting all this by calling you insecure, slightly mad, have senile dementia and need HRT.
Right.
Anyone who now says YABU is being deeply disingenuous and is obviously looking at this from the benefit of their normal, healthy relationship. Which is something the OP definitely does not have.
I don't even know how to go about advising you to rectify this, but 'leave the bastard' probably is about the gist of it. The fact that you have newborn twins with this 'man' makes it really sad.