What does it mean when a guy friend asked his friend to marry him even though they don't have a formal relationship that they just rely on their mutual feelings is he serious or just being sarcastic?

Thats only a question that you could answer. He either could be being sarcastic or he could be just throwing that idea out there to see how you will react because he is too afraid to share his true feelings. I would suggest bringing it up again if the opportunity presents itself and see what he says. If you have feelings for him then you should be able to ask him in a round about way.