The word, in Greek, literally means sharp-dull. And I believe, contained within the idea of sexiness, is yet another oxymoron. Or even several.
Sexiness is a little bit dangerous, a little bit risque, a little bit edgy, but if those things are turned up too much, then the safety goes away and the sexiness turns into something scary.
Sexiness is also a little bit comfortable, a little bit normal, a little bit safe. If the scale tips too far in that direction, toward safety, then the allure is lost and sexiness evaporates into mundanity. But somewhere in the middle, in the contradiction of safe danger, lies an aspect of sexiness.
Fading BeautyFor the most part the objective, physical beauty of each person peaks in early adulthood and declines from then. And yes, there are some objective aspects to beauty like symmetry and consistency. Often committed relationships start during this peak and then the commitment grows as the beauty fades. But what's almost magical about sexiness is that it can increase over time even though the objective beauty might be fading.
Sexiness is partly defined by objective beauty, partly by subjective beauty (what you, personally, find appealing), and partly by experience. Our brains use little chemical rewards to remind us what is good in our lives, things like dopamine and endorphins. When those rewards are triggered by your partner, your brain starts to see them as a source for the rewards and, here's the really cool part, it will kick out some of the rewards in anticipation. So the more attractive you are to your partner, the more attractive you'll be. Having experiences that make your partner attractive will make your partner more attractive to you.
Beauty wanes, but sexiness can wax greater for as long as you keep feeding your brain with positive experiences.
It's well established, both through research and anecdotes, that variety is sexy. That's why we can see someone new and be instantly attracted to that person. It doesn't mean we're being unfaithful to our partner to notice a new face, it means that our brains are working normally (of course what we do with that noticing is the important bit).
Our brains are wired to notice change. You'll see something that's moving first. You'll notice something that's different before you'll look at what's familiar. Those traits made humans able to survive in the wild, to notice predators, and to find food.
But our brains are also wired for consistency. We gravitate toward patterns, even to the point of creating them so that we can make sense of the world. That trait has allowed humans to develop language and science and math and art and engineering and music. The consistency of the patterns in our world have helped us to learn and grow beyond just surviving.
Sexiness is often seen in what's different. For someone you've never met, the very fact that you've never seen them adds to the allure. But for someone you know, especially for your romantic partner, sexiness is often seen in what has changed: a new hairstyle, new clothes, a new scent.
Here's the contradiction: the more familiar you are with a person, the less they trigger your difference-sense, but the more your pattern-sense. That, in turn, allows you to see smaller and smaller differences. Essentially, sexiness is found through the discovery and wonder of unraveling the ever-deepening mystery of another human being.