I suspect part of the reason the marraige thing/wrong body idea gets a pass for a few reasons.
One being that there is no real history of anyone proposing this kind of crazy idea, so it's never been ruled out. It's much harder to make a change on something that has already been rejected as incompatible with Christian theology.
There is also a hierarchy of decision making, the more important or central an idea is, the higher up it has to go for everyone to agree with it, basically. For some things that means the agreement of all the members of the Anglican Communion. With something really new like this, even if a lower level wanted to make a decision, if it was controversial higher up, it has to be resolved there first.
But this is just not a live issue in many of the more conservative parts of the AC, so what they are doing in the UK stays under the radar.
And I am pretty sure people like Welby really believe it is some kind of intersex condition. So they don't bother to apply any theological thought.