While gender discrimination is a problem, I think people are often far too quick to jump to the conclusion that pay differences between men and women in similar occupations are due to gender bias. The LW notes that she and her male counterpart have a similar amount of experience and seem to have a similar academic background. However, the quality of their experience may be quite different. In addition, her male colleague may have come to the company with excellent recommendations from former employers or professors.
Although I don't know the details of their occupation or industry, I think it's a bad idea to request raises when your only justification is that so and so makes more money than me. It's an even worse idea to imply as Prudie suggests that the pay differences are due to discrimination. That is a loaded word that immediately puts people on the defensive for fear of litigation. I believe the better course of action is to work hard, take initiative, and perform well on assignments, then use evidence of your performance to justify a pay increase. Employers usually respond favorably to this approach and in the rare occasions that they don't, if you are good at what you do, there are usually competitiors that would be willing to meet your demands and hire you away.