That said, I don't think you could write cross platform opengl code unless there was an abstraction. However, there is a framework called monogame, which depending on what you're trying to do, may be useful. See http://www.monogame.net/showcase/
I keep a lot of business logic and networking code in shared modules, then have the UI be very thin. These shared pieces can be tested on a desktop system. The biggest negative about Xamarin is it feels like they push some things out the door too soon, without sufficient quality or documentation.
If you have any questions, feel free to shoot me an email at curtis [at] saltydogtechnology [dot] com.