This professor seems super arrogant. Like it's fine to disagree with someone but (s)he's being so condescending. I'd probably be a bit cheeky and respond with something like this:

"I have no issue with vegans. It's your polemical zealoutry [sic] that comes across as puerile". You're responding to a post asking for recommendations for vegetarian protein alternatives, yet your response was to tell them to eat meat.

Eating meat may be justified if it were necessary for survival or health. This is not the case. According to the American Dietetic Association, vegan diets "are healthful, nutritionally adequate, and may provide health benefits in the prevention and treatment of certain diseases". Furthermore, vegan meals are typically cheaper than meals that incorporate animal products, so such a diet is not a luxury. Most of the world's poorest peoples treat meat as a costly luxury only eaten on special occasions.

Meat production is detrimental to the environment. Animals are capable of suffering, and by avoiding their products we can reduce their suffering. In the modern world, eating meat is morally unjustifiable and your polemical zealotry comes across as puerile.