If you're being sarcastic, it's not obvious due to the fact that some people really do believe that.
If you're being non-sarcastic, you're crazy. First, gender isn't "state-sanctioned", it's genetically determined. And y'know what? On 99.9% of people it's pretty damn obvious. Even the folks who go to all the trouble of dressing up to look like the opposite sex are usually identifiable at a glance.
Some folks do have serious issues about their gender identity, but I think Google+ is a better place without them.