...are related. In many cases, one causes the other.
Yes, there are some men who get uncomfortable knowing that the woman in their lives brings in a bigger paycheck and it threatens their self-esteem. They're a portion of men.
However, an even greater portion of men would flat out stop being bothered by their girlfriend making more if the average woman was not so hung up on how much her boyfriend makes to begin with. If most women did not care about a man's income, most western men would not get bothered by the issue at all. It could even become an accepted norm. The fact that many woman DO consider it a standard issue is part of why many men are "intimidated" about falling behind. Because the women they want to date will literally penalize them for it...
Them being related is part of the point that I'm making. The landscape has changed a bit over time but I'm not so sure that it's mainly due to women making their SO's income an issue. If a guy has a patriarchal "I must be the provider" mindset, he's likely to have an issue with it regardless because he wants to feel like he's the main one doing the providing. As long as women are expected to deal more with the home and kids, I'm not so convinced that them ALSO earning more money is likely to become the accepted norm. Men are going to have to be more willing to take on some of more traditional gender roles of the opposite sex.
I'd wager that men's desire to be the primary provider has more to do with women's disrespect and disregard for men they outearn. As has been mentioned, women don't tend to want to mate down, which is probably what puts pressure on men to want to be the provider. Women largely still want men who are above them in the social ladder and are loathe to settle for men under them. ---