Petitions, Petitions, Petitions

I see that Jeremy has requested us to sign yet another petition on Gay Marriage. I slightly dread petitions. I have never known a petition to have any effect whatsoever, apart from in school once where chips were removed from the daily menu and I successfully led a sixth form revolt to restore them to our diet. On the recent Gay Marriage debacle I didn't sign the Coalition for Marriage petition, nor did I sign the other 3 or 4 that were out there, but I did write to my MP and spoke to him regarding the issues. I have since written to him to thank him for voting against the bill.
My issue is this: it is not as if the government does not know the
strength of
opposition to Gay Marriage; they just don't agree with it and are
determined to
act against it. Even if we had a million signatures on the petition it
would not make a difference. That is not to say we don't write, visit
our MPs, lobby the
government; but can someone please tell me what good petitions do? I'm not sure supporting marriage entails signing yet another petition!