Nah, I've heard 'He' pronouns in game used for Mettaton. I think that Mettaton is male.

I mean technically, in a sense, he is genderless. I mean, since robots don't have genitals and all. But, he is also referred to as a 'he' by almost everyone in the underground. So, he isn't male, but he identifies as one. Even so, it should still be kept that he is male.