It's hard to visualise, yeah.

Let's say that demand for bacon fell by 50%. It seems obvious that the market would soon respond and supply of bacon (in number of pigs raised for slaughter) would also fall by 50%, right? Okay, so re-visualise for other values -- 40%, 90%, 10%, 5%, etc.

You should now be convinced that there's a linear relationship between bacon supply and bacon demand. At a fine enough granularity, it's probably actually going to be a step-function, because individual businesses will succeed or fail based on pricing, or individual farmers might switch to a different crop. But every non-consumer of meat is just as responsible for that.

In other words, let's say 5% of people are vegetarian, causing 5% less meat production. We're all equally responsible for that decline, so we all get to say that, on average, we caused fewer pigs to die.