Seventy-five years ago, another king couldn't take up the job because he wanted to wed a divorcée; no one's challenging Prince Charles' right of succession for doing the same. So if gay Christians want to borrow some traditions of marriage, why should the fuddy-duddy bishops stop them?
When the British government introduced civil partnerships, they failed to make provision for spousal titles. What if Prince Harry turned out to be gay and married a hypothetical male lover? Existing protocol couldn't cope with such an event.