Foreign Dual citizenship and marriage – what are the requirements?

Im looking to make a permanent move to Japan very soon and i’m confused by some requirements that have been given to me by immigration consultants and hoping to get more insights from the community!

I and my same-sex partner are both dual citizens of the UK and the USA. We were married a long time ago in the UK and have a UK marriage certificate.

We have been told that because the marriage certificate is from the UK, we need to use the UK as our “home country” (front of residency card) and apply with the details from that passport. And then when travelling in and out of Japan we would need to travel on the passport registered on the front of the residency card only.

Except we would like to use our US citizenship (name changed as part of citizenship). And if course if we travel from Japan to the US we are required to travel on our US passport.

Why would there be a requirement for the marriage certificate to be from a specific country? Marriages are recognized world wide…

Is there a more sane path here, or is this something that we just have to live with because “that’s the system”?

by nickwil