Common myth, both true AND untrue. People who say it has 100% nothing to do with Acne are completely wrong. Some people may masterbate constantly and not have any break-outs, and those are mostly the people who aren't prone to acne in the first place!
Masterbating effects hormones in your body. Hormones effect acne. Does that mean you shouldn't masterbate? Nah, I still do all the time, but I even personally myself HAVE noticed that when I did constantly, I had a little bit more acne. When I stopped for 2+ weeks it cleared up a lot, only to start again once I started again. It's all due to the fact that masterbation is tied in with hormones. You'll find countless people who have tried the same experiment and noticed a difference.
But for me personally, I'm at the point where I don't even care anymore. I know I will have slightly more acne if I masterbate regurally, but then I simply use things to fight acne. It's not like simply masterbating will cause a huge break-out. No it won't. But due to it's effect on hormones in the body, yes, it can perhaps (depending on the person) cause them to be more likely to have slightly more acne. If anything maybe just cutting down on it would help, but you don't have to stop entirely, that wouldn't really be any good.