The reason you were called on this kind of logic is that you were using the Principle of Indifference without justifying its use. Indeed, I don't believe its use is justified at all.
Wiki writes:
The principle of indifference is a rule for assigning epistemic probabilities. Suppose that there are n > 1 mutually exclusive and collectively exhaustive possibilities. The principle of indifference states that if the n possibilities are indistinguishable except for their names, then each possibility is assigned a probability equal to 1/n
Created by eternal being and eternal universe are not indistinguishable except for their names. Examples of things that are include the numbers on a die, cards and coins.
Now, the application of the Principle of Indifference is basically subjective ('
I judge that it is warranted in this case'). Many people would agree that it is useful when it comes to dice, but it can still turn out to be wrong (we later learn that the die is loaded). The argument that there is a 50% chance that God exists is an old one, and I've never seen any formal justification for it.
Does it being informal make it invalid? Not necessarily, but it doesn't make it illuminating or interesting either. And the case hasn't been logically demonstrated, it was not deductively determined.
Edited by Modulous, : No reason given.