The symbols are actually astrological in nature. The male is the celestial symbol for Mars. It is supposed to resemble a shield and a spear, because they're closely associated with Mars, the god of war. The female is the celestial symbol of Venus, who is the Roman godess of love, beauty and sex. There is less consensus over what this symbol represents, but one of the most popular explanations is that it represents Venus' hand mirror. Curiously, though, the symbol for Mercury, the winged messenger, is used to represent a virgin female. It basically looks like the venus symbol, with horns.
If you're old enough to remember, there was a series of books called "Men are from Mars, Women are from Venus." Now you know the reason why the book was called that.