I don't know how far your sister would want to take this, but Colorado does have LGBT employment discrimination laws that ban employers from discriminating against employees because of sexual orientation or gender identity (they actually have unusually strict laws--most states either don't have them at all or just have them for state employment). I think that certain LGBT groups have also started using Title VII of the Civil Rights Act of 1964 (equal protection under the law regardless of race, gender, etc.) in order to combat discrimination in the workplace.
Again, this might be farther than she wants to take things at the moment, but she's definitely protected under the law--especially if her workplace allows men to talk about women, but then had her written up for mentioning that she's a lesbian (that's something that could turn her case into one of obvious discrimination, since they're enforcing something on one group, but not another). If things get worse, she can always looking into the Equal Employment Opportunity Commission and see if they can do anything for her.