IMO what's bad is not meat consumption itself, which we were able to do sustainably for millennia and it was never really a problem. The problem is that now you're getting too much of it way too easily. Eating too much meat is a problem because it perpetuates demand for unethical mass-production of meat and livestock are made to suffer as a direct result.
Take a look at the global human population chart over the last few millenia. Things can seem sustainable when there are a million people on the planet. When there are 8 billion things are a bit different.