Taehan is probably aware that his home life is different because his dads are non-traditional but even if your family is different you don't always assume that it's the same for everyone else.
For example, my parents aren't married and as a kid I didn't assume that other people's parents weren't married, I assumed they were. Or the fact that 99.9% of people assume everyone is straight when we meet them until we're told otherwise.
Doyun is putting out big alpha energy, manual labour etc. (which again, his omega dad might not do) and made one assumption one time that Doyun never corrected.
So it isn't impossible.
Why was taehan judging doyun back in the day and always assuming he was an alpha given that his parents don't fall into that tall alpha tiny omega stereotype?