definition of sexy girl: when you see her, you start to think that she is the best
she can be blonde/redhair/blonde/whatever
if, all in all, with her face, body, hair/eye color and dress style she looks cool and desirable, she is
sexy. and you do not think about the other girls at that moment, she captures all your attention and your lust.
she starts to be your favourite: until you see the next sexy girl. and that next girl might be very different from previous girl, but when you look at her - you want her, that's the only criterion.
the problem might happen when you see 2 sexy girls at the same time in one place. and if one of them is blonde, and the other brunette,
You Might choose a blonde. but it all depends on the particular pair. next time brunette can win. hair color itself is not important at all.
that's my point of view on the subj