I was in the Post Office yesterday. I had a letter to send by Guaranteed Delivery so I had to speak to one of the staff behind the desk. I was, I hope, perfectly polite to her, said please and thank you, and probably did at one point or another give her a brief, friendly smile, just to let her know I wasn't going to be a pain in the ass to deal with.
I still present male, so that's perfectly normal, acceptable, non-threatening behaviour.
At the counter next to me a woman was handing in a parcel. As it was measured and the staffer (also female) was working out how much it would cost to post, the customer told some little story about a man who'd seen her carrying the parcel ... and ... well, to be honest, I have no idea how the story went. But the point is, it ended with both women having a little giggle, and thereby bonding through laughter and, yes, smiling at one another.
This is not a sexist thing. It's not social conditioning. It's just one of the ways women socialise and affiliate. Likewise, when a women is having her photograph taken, or takes a selfie she'll almost certainly be smiling - and if she isn't then not-smiling is a deliberate, conscious act, done to make a point.
Women smile more than men, just like - on average, and there's a ton of research on this - they talk more than men, using a greater vocabulary and with a more expressive vocal range.
And this brings me to the point that Alaina made about a non-smiling Facebook face 'flagging up' a transsexual ... I make a point of never making a negative comment about any picture any girl posts on here, especially on the threads like "You look fabulous", or "Before and after". I mean, really, the last thing anyone transitioning needs is another transsexual being bitchy at them. BUT ... there are so, so many times, when I look at another moody self-portrait, or see someone screing up her face into some kind of moody/angry/wacko expression when I just long to say, 'For heaven's sake, woman - SMILE!'
So many people would look so much more feminine and pass so much better if they just did what 99% of cis-women do, which is smile, learn your best angle, and then, if you see a camera, combine the smile and the angle to get the best result. Maybe that's sexist. Maybe that's oppressive. Maybe it's a terrible sin that most women want to look as pretty as they can and be as popular as possible. But that's just how the world is. So if you want to be accepted as a woman, why not accept the reality of female life and ... smile.
PS: It has also been scientifically established that a smile is a self-fulfilling prophecy. The more you smile, the more genuinely happy you feel. Honestly, what's not to like?!