Since people have different ideas of what is truly sexy, then there cannot be anything that is 'truly' sexy (using a Kripkean argument here of necessary v. contingent truths; that is, if it's not a necessary truth, then it is not true at all). It's all relative. It's all subjective. Some people say sexiness is inward; some people say it's outward. Women can surely have sex appeal for different reasons: the right clothes (or lack thereof); the right attitude; that certain je ne sai quoi.
I would like to proffer my own arguments as to the physical beauty of a woman (divided into four categories), but I think I'll save that for another time. I'll end with a biblical quote (seems anachronistic, but I like the passage):
"Do not let your adornment be merely outward - arranging the hair, wearing gold, or putting on fine apparel. Rather, let it be the hidden person of the heart, with the incorruptible beauty of a gentle and quiet spirit, which is very precious in the sight of God" (1 Peter 3:3-4).
"I like refried beans. That's why I want to try fried beans, because maybe they're just as good, and we're just wasting time." - Mitch Hedberg (1968-2005)
"Football is about if you want to run and fight for each other, if you really want to play that killer ball." - Robin van Persie, Arsenal FC