I very rarely see it, but occasionally. Only ever holding hands, never see kissing, and even those instances are... once every month or so? More rarely? See straight people holding hands all the time.
I once gave my boyfriend a hug (no kiss), and was subjected to a twenty-minute-plus tirade from a stranger on the train (and we're talking threatening, not just giving vent). My boyfriend didn't feel comfortable with any measure of public affection after that. That might have something to do with it.
London.