It is not sexist at all, let alone "obviously very sexist". Don't impute malicious motives to people like that, it's extremely rude.

Come on man, I think it's safe to say a tradition that favor's men over women is reasonably sexist, especially given the time the tradition established women were property.

I don't think Belgium's feelings will get hurt, besides wait until you learn about all the other things that Leopold II did.