Yes, they can! When girls have sex with girls, they don't have to worry about birth control-- but they can and should do things to protect themselves from STDs and HIV, both of which can be transmitted through vaginal fluids.

Protected sex for girls who sleep with girls isn't about condoms (unless they're sharing sex toys); instead, it means using other forms of barrier protection. The chances of disease transmission are increased if there are even tiny cuts in someone's mouth or on their hands, through which germs can enter the bloodstream. Dental dams, cut-open condoms, Glyde? dams and plastic wrap (the kind used in the kitchen) can be used during cunnilingus to cover the vaginal area. For sex involving fingers, latex gloves and individual finger coverings called finger cots can be used.

Of course, no matter who is involved, the safest sex of all is no sex at all...