Hmmm. I have a male guppy that is in love with a big fat balloon belly molly 3x his size. At least she's female. He's species confused, but your guy is species AND gender confused. Are you sure your platy is a male? Is the platy annoyed or showing signs of stress because of him? I don't know how big your tank is or their other tank mates, but this is my recommendation: 1) Do you have a female guppy? if not, or not enough, get another one or two. 2) Make sure there are places for the platy to get away- caves and plants. I have more male than female guppies currently. This is not ideal and causes the females to get worn out from their advances sometimes. I have a great piece of driftwood that is hollow. Whenever I can't find one of the girls, they are in the log hiding from the boys. They come out when they are rested. 3) Move the guppy to another tank for a few days. Rearrange all the plants in the tank and change some water. Leave the tank lights on and put him back in at feeding time. You will have completely changed his territory. Placing him in the tank with lights on at feeding time is disorienting, but he needs to be knocked down a peg in the pecking order and this will do it to him.
Good luck and keep us posted
