Of course, every woman prefers that the man she is with will be a gentleman. But blondes prefer it more than other women.
The reasoning goes like this:-
1. It is well-known that gentlemen prefer blondes . . .
2. . . . Except that a gentleman is a man who, if/when he is attracted to a woman, is attracted to the whole woman, not just some superficial feature.
3. Blondes get more than their fair share of suitors who are only interested because of her hair colour.
4. So, if you're a blonde, you can't help being more aware than other women that a part of your sex-appeal is just down to such a trivial thing as hair colour.
5. So, because it happens less frequently for her, a blonde is more likely than other women to prefer to be valued for herself, rather than for some superficial feature.