Are houris really female? If yes, how can we understand that?

It is widely believed that houris are female divine creatures who will be married with men in the Heaven.
Where exactly we know that they are female?
Can there be male houris for women and female houris for men?
Can "the virgin houri" also means man who never had sex before?
Also how do we know that they are not human? Answers based on Quran first, then Hadith will be appreciated.