Vegans and vegetarians can move along and enjoy their day. You're cool already, and off the hook.
Cows are ruminants. That's a group of animals that are specially adapted to eat nutritionally-useless grasses. That's their whole deal. If you're living in a pastoral or premodern farming society then that's great because you can't eat grass and you can eat cows, so it's free food. But instead you live in a society (insert meme) where we grow food specifically for cows then ship it to cows. Again, the animal that's specialized in eating things that have no nutritional value, so we're going out of our way to grow plants with no nutritional value, and then ship enough of it around to feed an animal anyway.
What does that mean? It means by whatever metric you choose, cow meat is worse than half as efficient as other common sources of animal protein.
Feed conversion ratios. Enough feed to make a pound of beef is enough to make 2.5 pounds of pork or 5 pounds of chicken.
CO2 per calorie. 1000 calories of beef costs 13.8 kg of CO2. 1000 calories of pork costs 4.45kg CO2. 1000 calories of chicken costs 3.37kg CO2. Also note lamb topping the charts, which will be a running theme. (Also an extra reason not to use broccoli as your primary calorie source, if eating 13 pounds of broccoli a day wasn't a good enough reason on its own.)
Land use per year per calorie. How much land did that 1000 calories take? You'll need 119 square meters for beef, 7.26 square meters for pork, or 6.61 square meters for chicken. Note lamb topping the chart again. (Also apparently prawns can be farmed super dense, that's something interesting that I didn't know.)
Why do sheep show up so high on some of these charts? Because they're also ruminants. Don't eat sheep either.
Yeah of course actual veganism is good, while fish&egg-only is only "slightly less bad". But the latter requires zero effort and change of habits, while the former requires a small effort. In a reply to someone who keeps postponing "going vegan" indefinitely, why not offer the no-effort, no-excuse solution that they'll barely notice if they start today? I won't shatter the laziness barrier through text, but I can point out a complete lack-of-barrier they might not have considered. We all do it to some extent no matter what, for example I've never seen anyone warn against the insecticide-heavy crops when trying to convert omnivores, because the step of going vegan is obvious and significant while preferring oats to rice just saves some insects. Still saying to a meat-eater "try eating rice and beans it's good, farming practices are a question for another time" is fine, even though the better thing to say would be "try eating oats and beans, growing rice kills too many critters please nobody fucking recommand the scourge known as RICE in this thread"!
The stuff I said about sentience was bullshit though that's fair.