When talking about beef specifically, the problem is that cows produce a disproportionate amount of methane, which is a disproportionately powerful greenhouse gas. If you don't want to give up meat but do want to make a dent in your emissions, even just eating pork or chicken instead is an improvement. There might be ways to reduce the emissions of cows in the future (different feeds, gut biome interventions, bioengineering), but right now these are science fiction.

As for the energy inputs themselves, the notion of grass-fed beef often gets trotted out as a way to make beef more eco-friendly, but in practice this mostly amounts to greenwashing. Most beef is not grass-fed, and most places in the world that raise cows are not suited to pure grass feeding.

And then there's the land use aspect. When we're clearcutting the Amazon in order to satisfy the world's demand for beef, we've fucked up.