At the risk of hijacking this into an oil thread, could the ultimate culprit be an oil not made for motorcycles? It is the only substance that comes in contact with the friction plates. Either that, or the carb float stuck, and gas flowed into a cylinder and past the rings, contaminating the oil, and gas is the damaging substance.
That having been said, let me tell you how I fixed the 125 I mentioned a couple posts back. Someone gave me a 10 year old 1981 CB125S with 380 miles on it. The original buyer never went to DMV, so the "title" was a certificate of origin. I did a carb overhaul, stuck a new battery in it, and upon starting, I found the stuck clutch. I had been using Honda HP4 in all my bikes, but decided not to in this case until the motor got broke in(per Honda's recommendation). I used the method I described earlier, but if the bike sat for a week, I'd have to do it again. Once the bike had 600 miles on it, I switched to the HP4. It maybe stuck one more time, and the issue disappeared.