Board Thread:Questions and Answers/@comment-39106664-20190420011352/@comment-38873660-20190501184324

Mettaton is male. He wears pink because Toby Fox is a Mario fan. The "Mario and Luigi" series inspired Toby to create Undertale, along "Earthbound". And, according to Roy Koopa, one of the seven Koopalings (the Koopalings appeared in "Mario and Luigi: Superstar Saga"), "real men wear pink". So, Mettaton wearing pink was inspired from Roy.