The reason why it will be a long time, or hopefully never, before they allow children to marry is because children can not fully consent. Even if they say 'Yes', they really can't understand what they are agreeing to.
Unfortunately there will be idiots who will point to people getting married in their early teens back in the 1800's as a reason why today's kids should also be able to marry that early. But back then lifespans were much shorter, and the kids matured much faster, and it was probably still a bad idea even back then.
Things are bad in America with regards to moral standards, but I think we still have a long way to go before we have to worry about NAMBLA becoming as powerful an influence as the gay lobby.
I think our arguments lose support when we try to heap all sorts of imagined future horrors on top of very possible near term horrors. When we start talking about people marrying their pets or their computers, instead of it generating more support for traditional marriage it undermines the basic argument.
Right now I believe it would be best to focus on polygamy, which most people still oppose especially when it is of the Muslim variety. By making gay marriage legal in a few states (and soon the rest of the country when DOMA goes down undefended by the Obama admin) then the only thing stopping polygamy will be that there are only two lines on each marriage certificate. The polygamists will by logical extension wonder why there can't be three or more such lines.
They are part of the gay lobby, always have been. Even the liberal wikipedia doesn't try to hide it. They are connected at the hip.