Based on what your write, this may be illegal sex-based discrimination: the pay differential suggests (but does not absolutely prove) that. You could contact the federal EEOC or your state's Department of Fair Employment and Housing to file a complaint; the agency can evaluate it and, if they think there maybe something to it, investigate. Your employer will then have a chance to prove there is some valid, non-discriminatory reason why your male coworker earns more; if they can't do so, however, and the agency continues to believe that there is discrimination going on, you may be entitled to compensation.