From my personal perspective, it is sheer silliness, especially on the part of (a good number of) men. While I don’t agree with it, I can see why someone would be hesitant to date someone earning less than them. But I am struggling to see that many tangible downsides of dating someone earning more than you do.
There are, of course, some potential downsides in every situation. But given that men (in general) seem to have no personal qualms about dating women earning less than them, i fail to see why a good number of them would be so vocally opposed to the inverse of it.