BMO is a boy. Although in all of the seasons BMO is portrayed as gender neutral character, so it's really up to you to decide. In season seven though in the episode "football" it is confirmed that BMO Is a boy.
Personally no idea.but,in Japanese voiceover version,BMO is voiced by very girly voice.(I think,by same voice actress with Power puff girl's Bubbles,Japanese version if course)So here we might have printed "girly BMO"version.