If you use anything made with/from animal labor or products, you are practicing the same form of morality, you've just changed what's acceptable from "murdering" to "enslaving", "robbing", etc.
It is also no more tenuous than arbitrarily drawing the line at bugs or bacteria. Thousands of innocent insects had to die to bring us a banana (--death from pesticides, from mechanical crushing during harvest, from geting squashed on the windscreen of the delivery trucks).
Here's a summary of a paper in Nature about Bacterial perception:
http://biosingularity.com/2006/06/04/how-bacteria-sense-thei...
Better still is the communication, one example is quorum sensing. Bacteria sometimes take little polls before acting, like little mobs trying to rile each other up:
http://en.wikipedia.org/wiki/Microbial_cooperation#Quorum_se...
To the bigger point, putting an ethical line in here somewhere always struck me as a bit of a sorites paradox, the endpoints are fairly easy (don't torture humans, don't worry about bacteria), but there's no obvious line in the middle where you couldn't make an argument to include or exclude just one more species, especially when you confront the staggering biodiversity out there, and start to see everything as more of a spectrum and less discrete.
I avoid smarter animals, like pigs, squid, and octopi (dogs, cats, monkeys, or ravens if they ever came up). Cows and chickens are fair game. This is basically arbitrary, but I don't believe it's any more or less tenuous than anyone else's system, we're all making a call with limited information. (As Thomas Nagel put it, no amount of human thinking reveals what it is like to live as a bat.)
How do we define "suffering"? Is it the same emotional experiences we call "fear" and "sadness", i.e., the ability to have the concept "I am afraid/sad"? That means the animal needs to at least have a primitive form of understanding and introspection to qualify (e.g., great apes, chimps, bonobos).
If we go to the other extreme, we must admit we don't have a reliable method for communicating with animals (famous "dog whisperers" aside). We can only infer "suffering" is taking place when the animal reacts negatively to a negative external stimulus. But bacteria respond negatively to a negative external stimulus as well. Sure, there's mind-bogglingly more complexity in a mammalian brain than a bacterium, but if we reduce "suffering" to the observation of a negative reaction to a negative external stimulus, we must say even single cell organisms are "suffering."
Hard to strike the right balance.
[1] http://en.wikipedia.org/wiki/Plant_perception_(physiology)
[2] http://en.wikipedia.org/wiki/Inducible_plant_defenses_agains...
I eat meat, but I do feel it would be more ethically right to avoid it. In the same manner, it would probably be more ethically right to not buy/drive a car, not use cell phones (conflict minerals), give more to charity etc. (So where do you draw the line at how much of your own money you get to keep for yourself?) Something can be ethically bad even if we do not draw a line in the sand (or, even if we do not as a society deprive someone of the freedom to do those bad things).
If I were a vegetarian, and drew the line at eggs or whatever, I would be drawing the line because a line would make it possible and practical to live as a vegetarian. (Similarly, the law has to draw a line somewhere.) Nature doesn't draw the sharp lines that our human languages do, but that doesn't mean we can't rank actions in terms of more or less ethical, it just means it's hard work.
