And I also think that he has no interest in whether or not Britain is a 'post-Christian' society (whatever that might mean)
It means that the influence of organised Christianity on society at large is now so marginal as to be largely irrelevant to the majority, as must be obvious to you from the fact that same-sex marriage was legalised despite religious objections, and that for many of us Christianity is now something we can simply ignore.
.. nor is he that worried about what the UK civil law has said.
Even so you are still bound by them whether you agree or disagree.
It is only in the last 25 years that the idea of marriage being anything other than between different sexes has become law anywhere in the world.
It is called social progress, and we should be glad of it when any forms of discrimination are removed.
We all know that legislation can be wrong or poorly created, abd can fall over a period of time.
True, but legislation can also be beneficial and socially progressive.
I suspect he believes that this is a flawed piece of legislation, whether it exists in the UK, the USA or anywhere else in the world.
You are entitled to your opinion of course, but where it is based on religious dogma that conflicts with progressive social attitudes then, and since the UK isn't a theocracy, your opinions carry no additional weight by dint of them being 'religious'.
I see that, sadly, an MP has died at a comparatively young age, and so a by-election is pending. It would be interesting to see how a candidate campaigning on a specifically Anglican religious manifesto would fare and until such times as such as yourself 'put up' as regards public policy by seeking and gaining a democratic mandate then the rest of us can, if we wish, simply ignore Christianity and consider its stance to be both regressive and irrelevant.