You may find that if you do keep only females one will change sex.
from the net (this is bound to cause discussion :lol: )
Platys don't change sex, but there are two times in their lives when they can confuse their owners in this regard. All platys are born looking female - if you have a look at some newly born fry, all of them will have the triangular anal fin of the female. As they get to about 1/3 adult size, some of them will start to show that they are males by developing the gonopodium (male-shaped anal fin.) If the tank is overcrowded or they are underfed, this might be delayed until they are almost full-sized. This is more noticeable with mollies and swordtails than with platys; sometimes these fish can appear female until they are over a year old, if they are in the same tank as a dominant male. When separated from the dominant male, they suddenly develop male characteristics.
The other time when they can get confusing is very old females. Sometimes an old female will stop producing fry and may even develop a gonopodium. She has not actually changed sex; she is now sterile and will not show male sexual behavior or father fry. This happens to some females but not others, and again is more common in swordtails.
(The last paragraph is what happened with one of my gold lyretail mollie breeders)