Beauty is a transcendental that is not encompassed by the term "sexy," and they are not synonymous with one another. Something that is sexy can be beautiful as well, but not necessarily. Just like the act of sex itself: sex can involve love and be an expression of love, but not necessarily.
And just because something that is sexy isn't beautiful doesn't mean that it's ugly. That is to say, "not beautiful" isn't necessarily "ugly." Something can have a certain magnetism and one can lust or desire something without finding it beautiful.
Beauty is a more generalized term that encompass a whole range of aspects. Sexy is more specific and personal. I find most women beautiful in various ways but only specific women are particularly sexy to me. A woman can also be very sexy but not particularly beautiful. Or a woman can be very beautiful but not particularly sexy to me.