They are girls posing as boys. They explain this to Usagi shortly (within a few eps, I think) after the plane ep with the Threelight's movie thingie.
But if you really want to be pedantic, the "disguise" forms can be either male or female (ie. manga Ten'ou Haruka), while the Sailor form has to be female. The only "exception" is Tuxedo Kamen, if you classify Sailor Senshi as carriers of Star Seeds.
"What you witnessed, never happened. And I was never here."
"izapii afamehkinam etokafenam" - Mahou Tsukai Tai
"Tenshi no Tsubasa yo! Watashi to Hikaru wo Izanatte." - Misaki / Angelic Layer