Haha!
But he actually has a male given name so I never got confused.
However there are people I know well here, or even speak to privately, but I have no idea of their gender.
It's always a bit embarrassing to ask after knowing people for quite a while, although if I still don't know after that long, why should it matter?
It's difficult for me to understand why or when people will take that as an offense as I have no concept of gender for myself or others. But I respect the fact it's important for other people.
Edit. Just realize it's kind of the case for you, although I don't know you that much so it's less weird.