MrMimizu, if you are talking about signing up the address used in the "From" header to porn sites, then you're signing innocent people up to them. The "From" address is nearly always spoofed, or used from a trojan infected computer. So, the address you use to sign up sends porn to someone that didn't request it.
If you end up signing up a child to receive that porn, and it gets traced back to you (which it sometimes can with enough effort) you may end up getting a visit from an agency you'd rather not.
Spammers don't give their real address; they give someone else's.