I think a better way of phrasing it is that I don't know that a god exists (as in, any god, I can be quite certain that the god of the Torah or Bible is too logically incoherent to exist). I admit I don't know. But that doesn't mean I should act as though one does, especially as I wouldn't know which mutually exclusive one it would be if it did exist.
The burden of proof is on one who makes a claim to knowledge, either that a thing does exist, or that it doesn't exist. The default state is agnosticism, or admitting that you don't know, not simply disbelief.
Edit: In fact, the OP's original statement seemed to be agnostic in nature, admitting that they couldn't prove that god didn't exist, but since they couldn't prove that god did exist either, that they shouldn't waste their time acting as though it did ('pretending').
It was only the believer who misunderstood them as seeming to claim that god definitely didn't exist, but then they got into a sidetrack discussion about the burden of proof, rather than just correcting the believer's assumption about OP's belief.