This magic little word, simply means "(the thing makes you) think of sex first".
Thus, anything could be "sexy" depends on the sex orientation of a person.
For example, if a guy says "that shit is sexy" when he's actually pointing at an excrement (because according to another definition of "sexy", by FazzyC, this guy could just be a moron), then you can easily tell that he suffers from Coprophilia.
Though, when you consider a person "sexy", s/he is not necessarily always the sex object of yours at the time.
For example, your mom.
Further more, non living things can only remind you sex.
For example, a sexy car is not fuckable, but:
a. it gets you sex
b. you can sex in it
c. unless you are a Autobot
Life sucks, even the most successful man can only get to sex with a very few of "sexy" women he meets.