Why are there so many girls
From Second Life Wiki
Compared with other virtual worlds Second Life has the biggest female population (approx. 50%). Most of the women that log on Second Life are in the 35-55 age range. They also log into Second Life more frequently than males.
However, do NOT confuse the population gender with avatar "looks", that is: Anyone can play any gender at any moment by modifying his/her appearance (In the Appearance window, look in the Shape / Body section. You can easily change your gender by selecting either Male or Female, located under the side buttons.)
Long story short: yes, there are a substantial amount of female players around; but be advised that you can easily switch genders at any moment!
That said and done, you have been warned :)