'Gentlemen Prefer Blondes' is the title of an extremely amusing novel written by Anita Loos in the early part of the last century (subsequently made into a movie starring Marilyn Monroe). The title references the simultaneously naive and world-wise pronouncements of the novel's heroine, Lorelei, and has made its way into popular culture, but is not a presumption based on any kind of anthropological, biological or sociological fact.
In fact, just as many men prefer brunettes, redheads or any other hair color/type as they do blondes.
I happen to be a blonde who did not start out as one, but who decided to give nature a hand in my late thirties (or, as I like to put it, I finally decided to stop dyeing my hair dark brown and go back to being a natural blonde....) As far I'm concerned it was a great move, simply because my hair color is now more flattering to my features than the color I was born with. However, I received just as much male attention when I was a brunette as I have done since becoming a blonde - so my personal experience also bears this out!