We most certainly do not sell email addresses or release them to our advertisers, so the spam your getting would not be from registering on these forums. As mentioned, it was probably because you posted your email address in a thread and a spambot somewhere harvested it. I would highly suggest if you want to give out your email address do it either via PM, or obfuscate it where only a human would understand. For example:
johndoe@hotmail.com
would become something like
johndoe NOSPAM (at) hotmail (dot) NOSPAM com
It's really sad people have to do that to keep their email free from spambots, but until a way is found to efficiently stop spam it will continue to be an issue.