I draw the line below cows. Below chickens. Below pigs, turkeys, sheep, goats, rabbits, and all the other animals on farms.
Below fish. I don't say that for shock value. The evidence - scientific and observational - is overwhelming that fish are sentient, feel pain, make decisions, and seek happiness. They appear to experience fear and anticipation, and in all likelihood they possess a wide range of emotions.
Don't worry about where the line is. You don't need to know exactly where the line is to stop killing animals who are clearly sentient, feel pain, have the capacity to experience joy and suffering, and have a will to live.
Where do you draw the line for assault? Rape? Murder? Any form of moral transgression has gray areas near its boundaries. But we don't defend assault, rape, and murder by saying "Where do you draw the line?" We don't commit grand larceny just because we can't precisely pin down where one draws the line for theft.
Try to be as kind, non-violent, and non-exploitative as possible and you won't be concerned about where the line is. Try to make the boundaries of your compassion as wide as you can.
I strongly suspect that "Where do you draw the line" is not so much a serious intellectual inquiry as it is a delaying tactic or a reflexive, defensive response. You're probably a decent person and to be reminded that you're participating in the wholly avoidable killing and torture of innocent animals hurts. It's normal to come to your own defense.
I would advise asking yourself honestly what it is that keeps you doing this and prevents you from moving toward a more compassionate and non-violent diet. Common factors are habit, ignorance, and fear.
Habit. You started eating meat, dairy, and eggs when you were a baby, and it's what you're used to. You practically have a "muscle memory" at this point: You go to the store and almost automatically pick up chicken breasts or ground beef and a carton of milk.
Ignorance. You don't know what you'd eat if you were a vegetarian. (The variety is almost endless - really.) You think we need to eat meat. (We don't and in fact, our bodies process it quite poorly.) You may think animals on farms are treated well (they're not). You may think cows need to be milked (they don't) and that we don't kill laying hens (we do, when they're quite young). You may think that farmed animals are killed swiftly and painlessly (it's actually quite brutal).
- You may be afraid that a vegetarian - or to be more precise, vegan - diet will be strange and unsatisfying. (With the plethora of good-tasting meat, dairy, and egg substitutes on the market, you can transition to an animal-free diet without changing your routine or cuisine much at all. But I recommend expanding your horizons or you'll miss out on a world of tasty flavors. A vegan diet is very satisfying both in terms of variety and healthfulness of food and the fact that you no longer have to engage in denial or unconvincing rationalizations for killing and inflicting suffering on animals.)
- You may be afraid of not getting enough protein. (You'll get plenty of protein, as well as other nutrients that you may be short on now, and you may very well lower your risk of heart disease, stroke, cancer, and diabetes. Unless your new diet consists of nothing but tortilla chips and Dr. Pepper.)
- You think you could never give up cheese. (Trust me. After six months of no cheese - and possibly much sooner - you'll see cheese as a big glob of unappealing, fatty, artery-clogging animal secretions, with added pus from mastitis infections. One reason is that cheese contains concentrations of casomorphins, which have a narcotic effect. Within a few months you clear your body of the cravings. But if you must, there are some good vegan cheese substitutes.)
- You may not see yourself as a "vegetarian" or "vegan." Perhaps you think that only pasty-face aging hippies or spoiled in-your-face radicals are veg*ans. (I say this with love and respect: Get over it. Be who you are, just a kinder version. Don't worry about what your friends think. You'll probably unintentionally make them feel guilty. Simply explain what you're doing with confidence but respectful understanding of their unease. They'll come around.)
I grew up a die-hard meat-eater. I had all the aforementioned habits, ignorance, and fears. I'm not Superman, I don't have super-human willpower, I'm not into eating bland rice and steamed vegetables every night, I'm not a great cook, I'm not into hackey-sack, I like jazz and I think the New England Patriots are having a phenomenal season. If I can become vegan, so can you. And when you do, not only will you not ask questions such as "Where do you draw the line," but you will probably respond to them just like I'm doing now. Do the right thing, and good luck. Remember, help, support, and information are two or three clicks away.