The majority of female rapists are child molesters rather than those who go out and have sex with unwilling adult males. The fact is that among adults raping adults, it is primarily a male-perpetrated crime, but it is possible and there are cases of it happening, and child victims of female molesters and rapists are even less likely to report the abuse than adults, therefore we don't hear about it.
I Googled "female rapists" and pulled these from the first few pages to show cases of
http://www.aftenposten.no/english/lo...cle1151867.ece
http://news.bbc.co.uk/1/hi/uk/1319211.stm
http://media.www.dailyiowan.com/medi...t-265923.shtml