When a machine character is depicted romantically or sexually with an anthro character.
For the purposes of this tag, a machine character can be roughly understood to be anything that would warrant the 'Robot' or 'Animatronic' species tags, but would not include a fucking_machine, milking_machine, etc.

Some ambiguity is still present here - protogen are tagged with Robot, but are rather consistently cyborgs. Should there be a separate tag for cyborgs? Is anthro_on_machine the best choice, given the existence of a 'machine' tag which is specifically *not* what's being referred to? It could do with some thought and work before anyone goes and tags a whole load of images with this.