>I would imagine that the majority of people who are underpaid are underpaid not due to discrimination, but due to poor negotiating ability, lack of awareness of the market rate, etc.

That is due to discrimination. Discriminating against someone who is socially awkward and can't negotiate isn't any different than discriminating against someone for their gender. Both factors are things people can't generally control. One might even go so far as saying it is worse than discriminating based on religion, given the general view of religion being a choice one can actively change.

Historically we have been fine with such discrimination, but it wasn't too long ago the true was based on all the forms of discrimination we now view as wrong.