I don't think this is uncommon?? I have had an Orange, High-fin Platy female for over 6 months. She is healthy, VERY colorful, and was full-grown (or close to it) when I got her. SHE HAS NEVER BEEN PREGGERS.
Both of the males I have had during the 6 months have either paid NO attention to her, or simply chased her away constantly. My Blue Mickey was totally color biased and hated Orange fish of ANY kind, so that explains him. The other one I have no explanation for. Apparently she is just not what they want for a mate.
I have also noticed that my Platys are monogomous for the most part. They will choose one female (sometimes 2) and not pay any attention to any others in the tank. I have no idea why, just the nature of the fish???
And, Yes, it's possible to have an infertile fish.....but how do you know
?
With Swordtails, it's easy to think a young male is a female. They are slower to develope then Platys or Mollys in my experience. But all of my Platys have been sexable by 4 months, tops. I had a Swordtail that I SWORE was female at about 6-7 months old. Then, in a span of about 2 weeks, he developed his gonopodium and his Sword and was a BIG BEAUTIFUL male. Patience is the key with them.
NOT my strong suit