i've been thinking for a while about what diets are defensible.
georgia ray has an excellent article in asterisk about which animals
and animal products are ethical to eat. the long and short of it is 
that jellyfish and sessile shellfish are ok because they don't think
or feel pain, and dairy is more defensible than eggs because it's
less work for a cow to produce milk than for a chicken to produce a 
egg. read the piece here: asteriskmag.com/issues/02/what-i-won-t-eat

the issue is that i don't think the problem of food ethics can be
resolved at the individual level. i don't eat meat, and i haven't
for about ten years, not because i feel it as a moral imperative on
the basis of some quantifiable metric of suffering, but because of
the well documented horrors of factory farming, for animals as well
as for the laborers and general environment. i'm also aware that
most barnyard animals i wouldn't be comfortable killing myself. i'd
be very unhappy killing a chicken, pig or cow. that said, i don't
eat fish either (excluding incidental anchovies or fish sauce in
restaurant food, which i don't go out of my way to avoid), and i
know from experience that i'm not bothered by killing fish. is it
fish's taxonomic distance from me or the fact that i can just let it
suffocate so the act of killing feels more passive? ray gets at the
issue of insect suffering, something i have no qualms about at all.
i love killing bugs. i think personal feelings of dis/comfort with
kinds of animal suffering are a poor guide to individual action---or
perhaps better said, individual action is the wrong register on
which to address the problem.

the problems with animal agriculture are social. they're produced on
a level vastly beyond any one individual's capacity to influence,
unless said individual is a policymaker or agricultural executive.
the harms they produce exist on a mind-boggling scale. i don't think
it is morally wrong to kill an animal for food in the abstract, nor
am i broadly opposed to the use of animal products. i am opposed to
the dominant form of animal agriculture in industrialized countries,
though, for four basic reasons:

1. it causes far more animal suffering than is strictly necessary
2. it produces work conditions that are exploitative and dangerous
3. it increases the likelihood of novel pathogens spilling over into
   human populations (whether viruses, superbugs, or prion disease)
4. industrial agriculture, especially animal agriculture, is a huge
   contributor to water shortages and global heating

i rank them roughly in ascending order of importance, but any single
reason is sufficiently disqualifying. the individual act of eating a
vegetarian diet is obviously insufficient to address any one, or to
remove the individual from a position of culpability. in turn, using
my case. as a vegetarian who eats dairy and eggs (and on occasion,
incidentally, fish):

1. i willingly participate in factory farming, even buying "humane"
   eggs and dairy. i know how permissive the certifications "humane"
   and "cage free" are.
2. even if i went vegan, plant agriculture is still exploitative and
   damaging to workers' bodies. farming in north america depends
   on captive migrant labor. the conditions campesinos work and live
   in are generally unsafe, unfair, and insulting to human dignity.
2. a. "family farms" and "co-operative" solutions do not solve this.
      as hamas publicized with their early hostage releases, israeli
      kibbutzim are mostly staffed by exploited migrant workers.
      i've seen first-hand the same is true of many "family farms."
      unless you get produce exclusively from farms where you know
      the conditions, or grow everything you eat yourself, you can
      assume exploitative conditions.
3. eating eggs but not chicken arguably makes me /more/ culpable in
   the inevitable avian flu pandemic, since laying hens tend to be
   more tightly packed in conditions highly conducive to the rapid
   spread of pathogens. simply buying "cage free" eggs offers little
   plausible deniability, as established above.
4. the broad ecological effects of animal agriculture are similarly
   not exclusive to it---soil degradation, deforestation, water use,
   carbon emissions, etc. soy, alfalfa, almonds, and countless other
   crops are culprits in these ecological catastrophes as well.

with even a cursory awareness of the problems of animal agriculture,
it becomes obvious that we are not dealing with an ethical problem
to be remedied through individual dietary choices, but an entire
sector of production that must be razed and rebuilt as a social
necessity.

so why do i remain a vegetarian? it's mostly patchwork, capricious
personal preferences. it remains the case that i could not kill a
chicken or cow myself without feeling gross about it. beyond this,
it's cheaper not to buy meat, and i do not make much money. i'm also
jewish, albeit not particularly observant, and vegetarianism makes
keeping a kosher kitchen basically automatic. not eating meat lowers
the risk of food-borne illness (but of course produce contamination,
particularly in leafy greens, happens all the time). ultimately, i'm
persuaded that reduction in per capita meat consumption, especially
in the united steaks of burgmerica, would mitigate the ecological 
and epidemiological harms of factory farming by simply reducing its
scale, though of course it wouldn't eliminate them, nor is any
individual's decision to avoid meat a meaningful contribution to 
this end. so i do not eat meat, and i have no plans to resume eating
meat, but to address the problems with animal agriculture requires
systemic solutions to remedy its social harms. ethical appeals to 
individual dietary choices certainly mean /something/ (they meant
enough to me to change my diet, after all), but they will not be
sufficient.