I'll concede that I would be less averse to this if all cash shop items were tradeable. I think in that case the circulation of gil would even out more than it can with a limited item like wedding bands. It drives business away from gil farmers, and gives more options to players on both sides of the fence. It would also allow for scenarios in which, for example, someone can't afford a $40 wedding nor the in-game cost, but might be able to supplement the gil they do have by selling a lower cost cash shop item in-game in order to then buy the wedding bands in-game. It's still all a bit too much like what is seen in f2p titles for my liking, but if we're going to go this route I'd rather open up the entire cash shop to this sort of trade than have it be limited to this particular instance.
That said, such a system still inevitably favors the wealthy in real-life. The terms of generating money are a little more fair in-game, whereas I might be able to spend what amounts to 15 minutes of RL work for 4-5 million gil, and that same amount is equivalent to 3 hours of RL work for another player.