Consumption of everything is the problem. More people means more pollution. Plastic is a side product of oil. Cement is one of the big players and it's used everywhere.

Eating meat on the other hand is not as bad of a reason as you'd think. People often think that cattle feed comes from same fields as out food, but that's simply not true. For the most part it's leftovers from food we consume, plant matter from corn and wheat for example, and from fields that would otherwise be unused due to low fertility. If you remove meat from the diet only thing you are left with is excess plant matter.