Been there. I was actually glad for my discovery, as I was wanting a girl, for one that was supposed to be a boy. At least you can leave him and Ollie together longer now, and don't have to separate them so soon, as long as they are getting along.
Buster (so weird to call him that now lol) actually mounted Ollie yesterday and thats what made me check (even though I know females do this too) and his little weenie was sticking out so far... I never got around to confirming he was female just because he hates being picked up, and I trusted her judgement.. So then I made sure to check Ollies gender too, hes a boy like they said. I dont want Ollie to get hurt so they cant be together anymore until they are both nuetered and I can start the bonding process. I hope Ollie doesn't get too sad about that, he loves "Buster"