While you can display your gender though alternatives such as status, signature, bio or plug-ins, those are alternatives.
Perhaps making this an option for post is a more complicated issue, but having the option to display it on your profile where your age, "member since" and "total posts" info's are displayed can be a great idea!
You could display "male", "female", "non-binary", or other options. Or not display at all.
If you have the option to display your age, I don't see why there wouldn't be an option to display your gender if you want to. It can be very informative.
Also considering the fact that you can choose a gender option in "My Profile", VIA the "Edit Your Details" option, yet it has no public use (I'm not even sure what is the use of it).