I approximately go by Bentham's criterion for what makes humans morally significant: we can suffer, as can animals. Rocks cannot. There is no reason to believe one configuration of a rock is "better" for it than another, as it has no kind of mind. I see no reason to believe a plant has any kind of mind either.
A pig, however, does prefer some states of reality over others, to quite a great degree. I think it's reasonable to say that the conditions we raise most pigs in mean their lives are a net negative: they'd be better off experiencing nothing than experiencing the lives and deaths we create for them.
I suggest you've tied together two questions. You're working backwards from "I'm going to keep eating meat", and have wound up at a conclusion that animals must not be morally considerable, because of that. Instead, separate the issue into two questions:
1) Are animals morally considerable? Is there anything I can do to an animal that is unethical? Is it okay to kick a dog, if I gain some momentary amusement at listening to it yelp?
2) How should the trade-off between my benefit and moral consideration to others work, exactly?
Link: boingboing.net/2011/06/30/richard-dawkins-on-v.html
Imagine a being so vast and powerful that its theory of mind of other entities would itself be a sentient entity. If this entity came across human beings, it might model those people at a level of resolution that every imagination it has of them would itself be conscious.
Just like we do not grant rights to our thoughts, or the bacteria that make up a big part of our body, such an entity might be unable to grant existential rights to its thought processes. Even if they are of an extent that when coming across a human being the mere perception of it would incorporate a human-level simulation.
But even for us humans it might not be possible to account for every being in our ethical conduct. It might not work to grant everything the rights that it does deserve. Nevertheless, the answer can not be to abandon morality altogether. If only for the reason that human nature won't permit this. It is part of our preferences to be compassionate.
— Albert Einstein
How do we solve this dilemma? Right now it's relatively easy to handle. There are humans and then there is everything else. But even today — without uplifted animals, artificial intelligence, human-level simulations, cyborgs, chimeras and posthuman beings — it is increasingly hard to draw the line. For that science is advancing rapidly, allowing us to keep alive people with severe brain injury or save a premature fetus whose mother is already dead. Then there are the mentally disabled and other humans who are not neurotypical. We are also increasingly becoming aware that many non-human beings on this planet are far more intelligent and cognizant than expected.
And remember, as will be the case in future, it has already been the case in our not too distant past. There was a time when three different human species lived at the same time on the same planet. Three intelligent species of the homo genus, yet very different. Only 22,000 years ago we, H. sapiens, have been sharing this oasis of life with Homo floresiensis and Homo neanderthalensis.
How would we handle such a situation at the present-day? At a time when we still haven't learnt to live together in peace. At a time when we are still killing even our own genus. Most of us are not even ready to become vegetarian in the face of global warming, although livestock farming amounts to 18% of the planet’s greenhouse gas emissions.
So where do we draw the line?