first, be careful not to generalize. just think about it, some girls can be exactly the same.
second, even after you have examined that and still think guys are more flirtatious, the reason may lie in the genetic code -- guys are simply more biological inclined to have multiple partners.
now, I won't go into the whole biology lessons here, but if you've learned about evolutionary biology, males are all about quantity while females all about quality. it is male's duty to inseminate and impregnate as many females as possible therefore propagate his genes throughout the population as much as possible. it is female's duty to find and stick to the most genetically superior male she can and have him impregnate her. females are just biologically more inclined to have one partner at a time because they can bear only one child at time.
however bear in mind, this isn't to say all men and women are like that. there's also the cultural overlay. humans are animals that are heavily influenced with monogomous culture. so in the end it's all up to the individuals.