Meat, and red meat in general, has got a bad rap because of various studies linking it with poor health (i.e. cancer and cardiovascular issues).
Personally, I would argue against it. We know from anthropology studies that humans only evolved to our current state because we started to eat larger quantities of meat. Fact is, there is no better source of nutrition and calories when you're looking at foods obtainable from the environment.
In the 20th century, we're in a different situation. Food and calories are easy to come by, to the point that many Westerners are gluttonous and we eat too much. We don't need to eat meat to obtain more calories, removing it as a requirement from our diets.
Vegetarians will argue that their diets have less of an impact on the planet, and that they're healthier overall. You can argue that point either way, but anecdotally, I've met few vegetarians that are actually "healthy." Many do not understand that if you remove meat as a source of protein, you need to supplement it with another source. Vegetarian sources of protein are not very protein-dense, so you need to eat a lot of food to get your daily requirement. Moreover, you need to ensure that you're obtaining all of your essential amino acids, which many plant-products don't provide. That's why Indians, a predominantly vegetarian society, will eat rice with daal. Rice lacks lysine, which daal contains. Together, they are a whole source of protein.
I've personally tried the vegetarian diet, and it's not for me. It's not for everyone. I found that to hit my protein requirements, I had to eat a lot of carbohydrates along with it, which meant a lot of calories.
Right now, I'm overweight, so I'm trying to hack my diet and lose weight in a completely different way. I eat absolutely no carbohydrates. All I eat is lamb (I'm Hindu, so no beef or pork), fish, and poultry. I also eat lots of green vegetables, nuts, mushrooms, and cheeses. Oh, butter and heavy cream too. It's called the ketogenic diet if you're interested.
I've never felt better. No intestinal issues or bloating. I'm losing weight steadily. And I get to eat awesome, healthy meals that taste amazing. And I would bet that my environmental impact isn't all that high. My grocery list is very simple, sticking to the outer corridors of the store.
It's not for everyone. But it works for me. I would bet that most studies that say red meat is bad, didn't control for carbohydrates. The biochemical processes that occur in the body in reaction to carbohydrate intake is likely what contributes to the issues surrounding red meat. Of course, few studies have been done to confirm this because of a general disposition that high-fat diets are bad.
In summary, people are against meat because they keep hearing that it's bad. They don't seek out alternate opinions, and would rather be ignorant than educate themselves.