Gerona has two blankets in the house that she LOVES to knead and suckle, and I let her do it because it soothes her and makes her happy.
Because she has her two blankets now, she no longer suckles on my husband's shirts (WHILE HE'S WEARING THEM, no less)!
So, you could try buying your kitty a soft blanket and use it a lot when you sit on the couch to it absorbs your sent and they may want to suckle that instead.
Some cats simply love to suckle. I don't think it's always a matter of, "Oh, they were separated from their mother too young," but rather that they just like to do it and it is a sort of therapy to them. It's why some teenagers still suckle their thumbs occassionally--it's a sort of subconscious soothing mechanism that makes them feel better. It worked when they were babies, and it works now.
As long as your cat is just SUCKLING and not EATING
the fabric, I personally don't see a problem with it except that you have a wet spot afterwards.
However, this is just my opinion on the matter.