Board Thread:Questions and Answers/@comment-27741771-20160311165848/@comment-4991168-20160311171933

Mettaton's physical traits and on-stage personality are those of a flamboyant metrosexual man, but his mental gender or sexual preference are uncomfirmed.

Since he was originally a ghost and became a robot, or so the game seems to suggest when going deeper into it, I think it's safe to presume Mettaton is asexual and biromantic or aromantic.