It is possible for some fish to change sex if there is no opposite sex, it happens in frogs and dolphins too. its a rare gene not remotely found in fish but since platys were crossed with swordtails to create colour some mutant genes may have developed. my mollys change sex from time to time but oncer thay change they will not go back to their original sex as they can only change once. Hermaphrodicey does not occur in fish but it occurs in snails and sluggs and some insects, but not fish. i am convinced your male has turned female, his anal fin may not change though so good look with him/her xD