I'm sure that sexual harrassment suits can be alleged against officers of the same gender, but it would be less likely to occur when compared to being screened by the opposite gender.

Typically, the phrase "How the passenger presents his/her self" is taken by visual means, referencing appearance and body language. I have seen instances where we have been incorrect in our judgement. Many times, you can't go by reading their names on their boarding cards, because many names can be used by both sexes. You make the best judgement call you can. Normally, what I'd do is ask for their license when presenting their baording card and read what gender is listed on the license. If I'm wrong, then I promptly report it to management. Hasn't failed me, yet.....

Everyone has the right to refuse being screened by an officer of the same gender. The only drawback is that you will not be allowed to proceed into the sterile area. Depending on the situation, it may result in LEO intervention.