They're showing off, probably trying to mate w/ the platies... not realizing that they're not guppies!
Have you ever watched a male and female guppy mate?
I've never seen actual mating, but I've seen my male guppies showing off their "goods" if you know what I mean... their gonopodiums stick out and that's how they get the sperm to the female (since ive never seen actual mating, I don't nkow if they inject it into them, or if it's free floating and somehow the female grabs it, I would think they inject).
~Tara