The key to health, in any kind of diet, is to eat a variety of foods and change all the time. If you don't want to eat meat, fine. But don't just sit there and chow down on tuna every day.
Fish have a tendency to absorb mercury, and eating too much of it can potentially be an issue. In moderation, there should not be an issue. But you still need protein. Soy has issues too. Too much soy, and modern research shows that it can have negative effects. So... VARIETY! Complimentary proteins. Try beans and rice every once in a while.
To answer your question, why would it be bad? You need to trust your intuition and eat what your body craves. Who cares what "classification" you are. Veggie, Vegan, Ovo-lacto, Pesco-veg, whatever! It is all about you. Forget what anyone else thinks. It's all a crock. Do what you think is right for you and makes you feel better. Don't label yourself. Just do it.