It's best to neuter them before that habit begins. Matter of fact Toby is at the vet's as we speak being neutered. I noticed that around 5 months of age he started a bit of humping. He didn't lift his leg on anything in the house other than the baby gate. But my feeling is that it's partially protesting at being gated when he wants to be free.
I think neutering/spaying is the best thing you can do for your babies. If you can't afford the procedure and need to do it at a lower cost you can try the ASPCA/Humane Society which charges about $35 for the procedure and another $15 for pain killers. You can also conta:
Muffin's Pet Connection, New York City Friends of Animals SPAY-USA
Good luck and I suggest at least using the belly band until you decide one way or the other. Even though your furniture is leather it will start getting damaged from the urine.