so gay animals eh?
You like?
Anyways,Animals do this for dominance, even if they have been sterilised.
My Dog used to do it, when my brother was layed down. He'd try to hump his legs.
He wouldn't do it to anyone else, just my brother, who, surprise surprise, wasn't able to control him.
If my brother game him a command (Like, Stay) he wouldn't listen, since my brother wasn't exactly telling the dog to do it with authority, if you know what I mean.
Anyways, Dogs (And most likely cats) do it to those who they believe are lower than them, which is why it only did it to my brother.
Also, what age were the cats? I'd imagine they stop doing it after a certain age, since none of my cats have ever done it to each other. They fight with claws, but they don't attempt to show dominance.