I recently had issues with an all male group of 4 platys in my tank, 2 reds and 2 orange and blacks. they each paired off 1 red, 1 orange in two pairs. every one was happy. Then one orange one got a lot bigger than the others and started showing constant aggression towards the non paired red platy. just wouldn't leave him alone.
It drove me mad watching this fish harass the poor red platy, who was not showing any aggression at all, just minding his own business. I mentioned it to one of the guys at the LFS I got them from, and he said bring the aggressive one back, and they would rehouse him. which I did, but then as soon as he was gone the other orange and black one has turned on the lone male red now... doing the same thing. weird.
It could be to do with the no females for them to take out their urges on or whatever, but I just didn't want females, and all the fry etc. TBH I wish I had never got them, I knew it was a bad idea... the mrs wanted them and convinced me. nothing but trouble, and won't be getting them again once they depart.
there is a lot of what looks like displaying going on, where the platys kind of hover side by side and their fins are all super extended, it doesn't seem aggressive. Its when one moves to quick, the other will go after it. or if the lone red crosses the orange ones line of sight across the tank... off he darts and chases the red one around.. a bit sad really, but I can't really ask the LFS to take another one on. so Ill see how things go. not my favourite fish.