I once had a male partner who earned less than me. I happily — and without gloating, ever — stepped in to help support him when he hit a rough patch. (As in, by hundreds of dollars a month.) I wasn't any less attracted to him at all — until he let his inferiority complex eat away at him and ramped up his abuse.

This is just anecdotal, of course, but I'm pretty sure that if a guy earns less but he is totally okay with the woman earning more, he's probably quite a delightful partner, and she still finds him attractive. By contrast, if he has an ego that's grounded in a need to feel "superior" to women — which is a quality that would turn any healthy woman off — this character flaw of his will expose itself pretty quickly once she starts outearning him.... and she will find the flaw very unattractive.

In other words: when a woman loses attraction to a lower-earning man, there's probably an explanation for this that involves their interpersonal dynamics, rather than her (ostensible) love of money.