Or Robin Hood?
Or Robin Williams?
Or Robin Thicke?
But besides that, even if Robin were a feminine name, since when are we only allowed to choose usernames which reflect our gender? Men use feminine usernames, and women masculine usernames, all the time, and there's nothing wrong with that. Especially on a One Piece forum; there's a lot of men whose favorite character is Robin. She's one of the best characters in the series.
Is the name MangoSenpai indicative of his gender?