In Peru, when you select the seat with Movil Tours, the seating chart shows you which seats are currently assigned to male and female passengers. Airlines should consider adopting a similar practice. Many people will disregard this information, but for others this could give them peace of mind. I can imagine a case where a woman may want to avoid being boxed into a window seat by two males.
Given airlines already have this information, it seems like it can't hurt to allow people to use it. In the best case, it can help mitigate the need for seat reassignment onboard, while in the worst case, nothing changes. Presumably someone could make an argument about passenger privacy, but to me this is moot because you're going to be able to see the person's gender when you sit next to him/her.