Based on what you said, the guy didn’t ask you why you thought he was married because he was trying to lie to you — sounds to me like your presumption that he was married because he has kids offended him.

Is it okay to assume a woman is married because she has kids? No, not really. Not really okay to assume it about a man either.

Given that you “found out” about him being married tells me he wasn’t hiding anything — he was trying to prove a point but you walked away rudely (according to you) so you proved another point.

That’s my inference based on what you wrote. I have done similar things to public school administrators that assume the mother is the primary caregiver.

Stop making assumptions and you won’t have conversations like that.