Demosthenes2:

Almost any reason is sufficient to justify different pay (as long as it’s not explicitly something barred). Given the market value here of an intern, I’d be inclined to take a different tack than a complaint.

This is absolutely correct.

This young lady may have think she's assessed their skills, their merits, their talents, their grades, etc., and found them to be exactly equal except for their sex (and they may be) but the fact is the employer can elect to pay one employee more than another for something as ephemeral as the perception that they would be a greater asset to the company by their manner and carriage, by the way they present themself to clients and customers. Does she really think that they'll come right out and say they pay her coworker more because he's a man? Of course not. They will have a perfectly legitimate explanation and she will be branded a troublemaker from the get-go. She needs to chart her own course and stop comparing herself to her coworkers before she drives herself and everyone else crazy. She probably had an older sibling that she competed with because this sounds just like, "well if Marcia got one why can't I have one?"