Well, we assume it's a nod to the royal wedding. The sign doesn't specify whom the 'couple to be married' are.
Visit them soon before they're covered in bird crap...these posts are normally prime perching spots for the Royal Park's avian population.
By the way, we should add that we haven't personally verified this. It could be a very good photoshop job. So let us know if you're walking past.
You can see more of Nagel's work all over town. We mapped his mushroom installations last year.
Thanks to Michelle Dayne for the pic, taken by Gavin Surgey.