My friend, Undertail is not lacking in male content. I cannot go five feet without getting ASRIEL nudes hucked at me some days which is a shame because he's like eight. Asgore's a new favorite of the bara community, as well. Mettaton doesn't get as much R34 as the other MALES because he's a very girly man and also already humanoid and also a robot and also a jerk specifically in all the unsexy ways.

Also a lot of R34 artists specifically seek out the inappropriate, and Mettaton almost literally asking for sexy fanart makes him the least appealing target for them.

Mettaton: IDK Alphys, why am I a rectangle?