I think most couples would be pretty offended if you called the girlfriend a prostitute for letting their boyfriend financially support them.
>Every study I've seen shows that women are paid less in than men in equivalent positions.
Women are paid the same as men in studies that control for other variables. I recommend you stop reading "studies" that don't control for things like hours worked and choice of career field.
http://www.consad.com/content/reports/Gender%20Wage%20Gap%20...
In fact, single childless women now make more than men in most US metropolitan areas: "Young women in New York City, Los Angeles and San Diego making 17%, 12% and 15% more than their male peers, respectively." (http://content.time.com/time/business/article/0,8599,2015274...)