From where did you clicked? The forum already have its own efficient proxy system, it don't need to rely on an external one, and there's no need for user to use one.
You mostly want a 100% answer? You're out of luck in this case. None one can give you that, regardless of the site.
You will be mostly save, I assume, if you didn't download anything or the site didn't run any malicious script. And no, I can't say and I won't look into it, if they do.
I think, that with only browsing the site, the chance are low that your PC catches something. But it is the best answer I can give you. Also that depends a lot on your own behaviour and setup.