Vegan or vegetarian? The two are different. Vegans don't eat ANY animal products at all (eggs, milk, etc). Vegetarians only restrict their diet to not eating meat ... and some of them DO eat seafood.
My sister was a vegatarian for over 40 years. The only reason she started eating meat (chicken) last year is because she has developed serious food allergies, including eggs, gluten and dairy. So she couldn't get enough protein in her daily diet, was losing muscle mass, and starting to feel weak. She could have upped her legume content and nuts ... but she has to lose weight for medical reasons, and these protein sources are fattening. Tofu is a complete protein, but she has had thyroid cancer, and too much tofu consumption is implicated in thyroid cancer, so she avoids tofu now.
Anyway, after over 40 years of meatless eating, she has added chicken to her diet. And feels better for it.
Her 38-year-old son has never eaten meat. Don't know how much longer that will continue, as he is developing a casein-intolerance (a protein found in milk and dairy), and he relied heavily on cheese for his protein.
Seems that the more you restrict your food choices, the more likely you are to develop intolerances to what you DO eat. Certainly, though, the red meats ARE good to avoid.
btw, my sister used to teach vegetarian nutrition, so it's not like she doesn't know what she's doing with her dietary choices. Her husband has become pre-diabetic, and she has been able to lower his blood sugar, without requiring insulin usage, through diet-control alone.