An interesting series of posts on vegetarianism over at NRO's Corner started with this one. Mark Krikorian argued that vegetarianism is immoral because it rests on an assumption of "the principle that animals are morally equivalent to humans."
I'd say that there is no need to impute a subscription to this principle in order to establish immorality of a diet devoid of animal products. At least I can do this for situations of a relatively narrow scope:
I think It would be immoral to abstain from eating animal products if it can be shown that to do so is to introduce higher levels of risk of substantial health problems for those that do not have the power to make such decisions on their own, but are dependent upon such choices of others.
There is ample evidence that embryonic development, and child development are very sensitive to levels of various nutrients, amongst which are several (like vitamin B-12) that have as their best dietary source so called 'whole' foods, and in particular meats. This is not to say that supplemental sources cannot be found for these things, but such sources are generally considered less reliable than 'whole foods.'
So, if you are responsible for a growing human life, and knowingly take on such risks, I would say that is an immoral act.
This matter is independent of whether or not you think animals are on the same 'moral plane' as human beings. That is an entirely separate issue.
I'm getting a bit peckish!
No lamb, unless it's in a gyro. Oooopah!