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This more of a dominance issue than a sexual issue. Even two females will do it. They are trying to show one another who is the superior cat in the house.
There is nothing sexual about it and they are not trying to mate. The humping is saying - hey, take that! I'm the boss! - It's perfectly normal.
This will ease with time and as the hormones settle.
Rather than scold, I think that you should try to distract the humper. Use a favorite toy or a feather wand. I don't think that Toby's actions are going to upset the others that much. If he bothers them too much, they will let him know.Let me know if you still have questions.