I'm surprised that no-one has brought up any arguments relating to environmentalism. It starts to get tricky because the sheer mass of current population almost puts the situation under the precautionary principle.
The environmentalist standpoint is the strongest standpoint in terms of an aggressive vegetarian. All arguments associated with 'freedom of choice' and 'past history of eating meat' are considered invalid due to the fact that environmentalism often requires sacrificing or adding more work into our daily lives than usual (recycling, proper waste disposal, not driving your hummer), making diet just another inconvenience.
I suppose if you subscribe to that method of thinking, then you should really only be vegan, because using animal products still require you to keep animals alive and producing carbon emissions and toxic wastes.
A simpler version of my argument is as follows:
Meat is unsustainable due to the emissions and waste associated with live animals, therefore we must stop eating meat. If you disagree, are you willing to bet 6 billion lives on it? Because 6 billion lives are at stake for a simple personal preference of 'waah i like meat don't take it away'.
I'm not actually a vegetarian, but I've heard the above argument so many times I'm curious as to what other people will say.