Gambling is like drugs and prostitution in that banning it just doesn't work.
It is a fact that gambling (internet or otherwise) is much more widespread in places where it is legal than where it isn't.
This could be applied to almost anything, though. It's also very likely a fact that prohibition of alcohol reduced overall alcohol consumption. Practically anything would be less common if you criminalized it. The issue is that it doesn't go away regardless of what you do, and creating the black market is substantially worse than what would exist if a broader, but safer market existed instead.
In this case, however, I'm not convinced that argument holds for online gambling. I don't really think the illegal market for online gambling is all that much of a social detriment, or that the consequences wouldn't be any less severe if they were made legal.