I'm not sure that's really as much of an income gap as you may think. You are taking modern assumptions, that a woman can succeed in any career, and applying it to a society that it was arguably less true in. It's entirely possible the nurse or secretary of twenty years ago might be a physician or executive if brought up twenty years later.
Additionally, I think you are conflating education with social standing, when they are somewhat different, and there have been different levels of social pressure against doing either at different times in history.
I agree that females who would have become nurses and secretaries in the past are now becoming physicians and executives, and without doubt this a good thing. My observation is just that by solving one problem (limited opportunities for women to become professionals) we inadvertently encouraged another (assortive mating on the rise which reinforces inequality).
It's also interesting you mention social pressure at different times in history. I am working my way through Downtown Abbey at the moment and it reminds me that throughout history there have been strong pressures for people to marry within their own class/caste systems. Probably for many reasons, but one of them is certainly the maintenance of wealth and power, which has typically been far less equal throughout those historical periods than the US is today. The post WW1 period in the US has been the exception not the norm.
I won't dispute that there is some level of extra assortive mating going on because of more equal opportunities for women. Even if it was just due to more opportunity and exposure, that makes sense. I do question whether in the past if the physician that married the nurse was really providing any sort of opportunity for status change (in either economic or social standing) in most cases, since I imagine the nurse and physician probably came from similar social standings, and thus likely similar economic standings.
If the nurse or secretary of 20 or more years ago was "from a good family", what change was really spurred by the union?
Additionally, I think you are conflating education with social standing, when they are somewhat different, and there have been different levels of social pressure against doing either at different times in history.