They say BMO is a boy in the show but it is a korean woman that does the voice for the BMO and she doesnt make any effort to add masculinity to the character (reference Bart Simpson) so in my book, BMO is a girl.
Look people have there own opinions I respect that but just beacause there voiced by a girl or a guy doesunt mean they have to be a girl or a guy. Like Naruto from Naruto in English dub he's voiced by a girl but is a guy. In bobs burgers the mom and Tina are both voiced by males but that doesunt mean there guys. So it doesunt matter. I for one think BMO is genderless. Just a robot. I respect your opinion though. But what you said is wrong.