Of course, and that would have been absolutely fine. Democracy is to be able to campaign for what you believe in.

But equally, politically, and democratically, it is not tenable to call a referendum (which is something quite rare in the UK) and then to ignore the result. So the reality is that the government and Parliament had to abide by the result even if the referendum was legally not binding.

Now people are of course free to campaign for the UK to join the EU.