I got into a conversation with someone on here in which they asserted that because you can use a passport to get married instead of a birth certificate and because changing the gender marker on a passport does not require a GRC, you can get your marriage certificate to match your "acquired" gender rather than that on your birth certificate without a GRC.

I don't think this is true given that both the government and trans activists say that this is not the case. According to both, we need a GRC to get married as our "acquired" gender, even if our passport has been updated.

However, I am aware that this was also said about death certificates and there has since been some evidence to the contrary, for example:

https://transsafety.network/posts/you-dont-need-a-grc-death-cert/

https://transsafety.network/posts/coroner-confirms-grc-death-cert/

This got me thinking. What were to happen if a British trans person without a GRC were to get married using their updated passport? What is the actual mechanism to stop this from happening? Is there some sort of declaration you make when getting married which would cause this to be fraud?

Edit: to be clear, I'm not being rhetorical, I'm asking literally, what would happen? How would it play out?