Orthodox can be vegetarian, but if we choose to do so it must be based on correct views rather than heretical ideas. Just as monks abstain from meat, laypeople also may abstain from meat as an act of ascesis. In my case, I was never raised to hunt or kill my own food, so the thought of doing so seemed to me to be unnecessarily violent when we live in such a time that we are surrounded with grocery stores filled with plant-based foods. If I did not want to kill an animal and prepare it for eating myself, I did not think it right to pay someone else to do the dirty work. Adam and Eve did not consume meat in Paradise, and at the time I stopped eating meat I found the argument compelling that our intestines show us to be much more similar to herbivores than carnivores by design. Eating less heavy and more plant-based foods may be helpful in the struggle against the flesh, but it may also be a matter of conscience if one learns about the horrors of the factory farm industry and cannot afford humanely raised alternatives. So, that is to say that there are many good reasons that an Orthodox person can choose to stop eating meat.
It is heretical, however, for an Orthodox person to consider meat eating to be "sinful", to in any way judge or look down on those who eat meat, to equate the killing of an animal for food to the murder of a human being, or in any way to elevate animal life beyond what God intended or to denigrate human life below what God intended.