Monks don't eat meat, which is understandable, because people can get all the nutrients that people lack from vegetarian food. But if you don't get married, won't mankind become extinct?

You are assuming that after death, life will cease to exist. In fact, a person who practices Buddhism diligently and never gets married will be born to the Buddhist land he wants to live in after his death. He's not dead. How can he reduce it? In other words, human beings are not extinct, but "migrating and immigrating". If everyone can live in the pure land of Buddhism and stay away from this dirty planet, what's wrong?