It’s reasonable to suspect that something like this might be important and denying him the truth is denying him agency to choose.

It may be the case that you would rather not know. How can you guarantee the same about another man you’ve never met?

Except if he finds out later that she was hiding this from him deliberately that could be excruciatingly hurtful. Whereas telling him now, I think, is more likely to play out as a “I just found out but you’re still my dad regardless of genetics” thing.

I think it’s more likely to be history homework for a North American school completed by an immigrant from HK/Taiwan.