You know Darwin as the father of the evolutionary theory, but you probably wouldn't go to him for dating advice. But you're missing out, because dating, mating, and procreating are all elements of the same evolutionary process. It's all about how we're wired.
Sexiness is basically the combined effect of characteristics the opposite sex is innately programmed to see as "markers" or "signals" for good genetic fitness and reproduction.
Darwin's sexual selection theory explains this in depth, but the overview is this: We are programmed to look for certain signs in prospective mates that help us discern whether or not they'll make a good partner—these signs are physical as well as behavioral. These biological predispositions shape our preferences for a mate irrespective of whether we actually want children.
Men and women look for different markers because our biology is different. Here's how to use this information to become sexier to the opposite gender?