Board Thread:Questions and Answers/@comment-28617495-20160601073159/@comment-27701762-20160601103518

We tend not to think about gender specifically until we are met with it. So when we go through with Mettaton, we see a gray box, and don't really think about its gender until we encounter something that gives us a clearer indication. The EX has some vaguely feminine features, so some might think "girl" rather than "boy" at first glance. Couple that first impression with a pink room, and it's less "I have evidence that Mettaton is a girl, and not a boy as other people say," but "I myself have come to the conclusion that Mettaton is female." Likely any other evidence (i.e. the fact that the game refers to Mettaton as "he" without fail) is then shunted to the side in one way or another.

It's no different than various people thinking that Alphys is male at first glance, and then refusing to accept that she is in fact female for any number of equally foolish reasons.