There is nothing to stop anyone from carrying out a "marriage" ceremony as long as the participants all understand that it has no force in law. It would be illegal only if there was a deliberate attempt to mislead the couple into thinking they were legally married. That would be a different crime, one of deception, not a crime of "illegal gay marriage". If I go out on a stag night with a fake policeman's helmet on, clear to all that I am a reveller, not a bobby, I am not committing a crime. If I wear that helmet with clothes that look a bit like a policeman's uniform, and go around trying to arrest people, then that's a crime. The element of deception is everything, Keith.