The Gmara (Yevamot 65b) says that the reason why men are the only ones that have to keep "פרו ורבו" is because right after it its written "וכבשוה" (the human has to take over the world), and men are the ones that do war. Since taking over and "פרו ורבו" are written together it is understood that men are the only ones that are obligated in them.
My problem is that the pasuk says "פרו ורבו" in a plural way and the only people that lived then by the Torah were Adam and Eve. This means that if Hashem would of want to tell this to only the men (then only Adam) he would of say "פרה ורבה"!?