Sex never goes out of fashion. The survival of the human race depends on it, for starters, so like sleeping and eating, sex isn’t going anywhere. But sexy? Sexy is different. Sexy has been kind of off-message, fashion-wise, for the past few years. Sexy has been uncool. But lately sexy is, in the words of Justin Timberlake, very much back. The milkmaid dresses of summer are being mothballed in favour of corsets worn under your blazer (see Alexa Chung in a corset dress under leather jacket at Miu Miu’s Paris show) or thigh-high boots with your miniskirt, as seen on Kate Moss on the Tommy Hilfiger front row in New…