besides the fact that a woman that can talk about fusion is not necessarily smart, I think that the point is not that smart implies sexy. But sexy does imply smart.Quote from deviltrader:
so you're telling me that if you had a discussion with an ugly woman about nuclear fusion and plasma physics, you'd get horny and want to bang her right afterwards? ok be my guest. but i'll take the gorgeous stupid woman since you don't want her.
A dumb expression is indeed unattractive.
Maybe we can replace the expression "is smart" with "has personality"