A cat's natural instinct tells it to scratch.
It's something they can't help doing and they don't do it to be naughty or to watch you hurl inanimate objects across the room! I once saw a grown man throw a workboot across the room at a cat and break a glass coffee table in the process.
Cats are going to scratch, of course: they don't want to have anything to do with those carpet-covered scratching posts you paid an arm and a leg for. If you haven't tried the squirt bottle method, it really does work: when you're at home anyway.
Each time your cat begins scratching the furniture, give him a squirt of water with a spray bottle. It's great fun watching them run away like a madman, and it also teaches them that something bad will happen to you if you rip up mommy's furniture!
If you are due for new furniture, here are some tips to keep in mind when choosing it:
Velour type materials are very hard for cats to scratch. The underlying material is quite tough and the most they will be able to do is get a tiny hole in it with their claws. My cat ripped to shreds our brocade sofa, but once we replaced it with velour type sofas, the furniture was fine. Of course, he tried to scratch a few times at it but got no satisfaction. As for wood: cats love to scratch at wood, but only if it is bare, or has a ridge to it, like a decorative raising in the wood. If there is something for him to catch his claws on, he will scratch it. Look for furniture that has straight wooden legs as opposed to those with decorative carvings.
Try placing some aluminum foil where your cat scratches when you are not home to perform water gun duty. The aluminum foil usually scares them. It may take some time, but you can train your cat to behave with the furniture and if you want to give him something to scratch, try a cardboard box, it's cheaper and they actually do like to scratch them!