Yes, you can! The world is dynamic, so are people. A lot of couples live together these days before marriage and have their houses well stocked.
Others want to bag pack and travel the world, so they don’t need gifts. Some couples do not even create a registry but make available cash payment methods.
So yes, you can pay cash and it is very convenient. The best is if you’re paying directly into an account or towards a cause like honeymoon packages. Do well to send it in before the wedding.