In theory, masturbation could be a crime though, it isn't in most places that I know of. But it could be. (Not that I'd really agree with it just saying that it's not something that could *never* happen.)
I mean I think sodomy is still *technically* illegal in the UK. (I'd have to fact-check on that though, but at the very least, at some point in the past, sodomy between two consensual adults was illegal there.)

As for is it a sin? Well, it depends on how you interpret your religious texts, or what you were raised to believe. Some people think it is, some people think it isn't. I'm pretty sure with Catholics, at least in regards to male masturbation is considered a sin because it is 'spilling the seed.'

I don't know much more on the subject, but I don't think there's anything wrong with it. Studies have been shown that it can help reduce stress, makes people feel better, and can even (for men) reduce the risk for prostate cancer.
I mean, as long as you're not doing it in front of a crowded room of students or something.