That's becasue, duh, they tend to work the hours that men do and generally, like men, are not lost from the workforce because of children---so they get paid like men.
But Economist magazine is "puzzled":
The magazine reports:
Marieka Klawitter of the University of Washington looked at 29 studies on wages and sexual orientation last year. On average, they found a 9% earnings premium for lesbians over heterosexual women, compared with a penalty of 11% for gay men relative to straight men. This discrepancy has been borne out by research on America, Britain, Canada, Germany and the Netherlands. Even after adjusting for the fact that lesbians are on average more educated than straight women, and less likely to have children, the gap persists....And I'm not sure where the idea comes from that lesbians aren't more aggressive than straight women, my personal experience is that they are. Their competitivness (and toughness) is why they have all the female talk shows, Ellen Degeneres, Oprah, Rosie O'Donnell and Suze Orman.
[A] working paper published last year found that whereas gay men behaved less competitively than straight men (accounting for roughly two-fifths of their earnings penalty), there was no such difference between lesbians and other women.
Lesbians may not need to behave differently to be treated differently. They could face positive discrimination, if employers promote them on the assumption that they will not have children and so devote more time to work than straight colleagues.
And it appears that it is a well known in the LGBT community that lesbians are tough and competitive; SEE: Andrew Sullivan and Rick Fitz.
Bottom line: There is no sex discrimination against females. If a female's marginal productivity equals that of a man, she gets paid like a man.