does not necessarily mean the person being referred to has a gay mom but is more of a easy insult that usable in any situation and can only be beat by no u or ur dad lesbian
bob: hey look its the retard joe joe: ur mom gay bob everyone: oooooooOooooooooOoooo
"Ur mom gey" is one of the cheapest paybacks in the entire world. You usually use it when you are in the younger age and dosen't find anything relevant to say in a conversation, such as the age of 15.