This is why my wife and I got matching tattoos. I have a tattoo of her butt on my butt and she has mine on hers. Its cute to us but big enough that it would be very off putting to others if either of us tried to cheat. It also cost pretty much nothing because she's a tattoo artist.
I highly recommend anyone looking to marry to work with their partner to find your own personalized symbol. It will cost less and bring you closer together.