Even if you are a monk, you are not necessarily expected to be a vegetarian. For instance, in parts of Southeast Asia where monks still make their morning alms collection, Buddhist monks are expected to eat whatever is placed in their serving bowls. So they are not allowed to pick and choose whether to eat meat or not.
As a matter of fact, the historical Buddha was NOT a vegetarian. One of the most famous incidents of his eating meat was The Buddha's "last meal" a plate of tainted pork, which lead to the Buddha dying from food poisoning. It is said that instead of chastising the villager who gave the food, The Buddha instead thanked the villager for providing this opportunity to seek release from the world and obtain Nirvana.
Further, the Buddha is quoted as saying that it is not a sin to eat meat, but it is a sin to eat meat from an animal that has been slain specifically for your consumption. I know
a lot of people who don't understand that, and when I asked some monks about this, they explained that even if all you eat is vegetables, then animals will be harmed. "Think of the pesticides that are sprayed on fruits and vegetables we consume that kill various insects and other pests. Think of how eating lettuce and carrots means that rabbits will starve to death."
In the Theravada school (which is the main school in my home country of Thailand), it is very hard to find any vegetarian restaurants or vegetarians. There are certain times of the year when one might be inclined to practice being a vegetarian. There is a popular festival in Bangkok's Chinatown district that is devoted to the Bodhisattva Kuan Yin, and many devotees will give up eating meat for the duration of the festival (about ten days, usually around October).
Another time when you are likely to see the largest number of Thai Buddhists take up vegetarianism is in the period known as "Khao Phansaa," which is sometimes translated as "entering the season" or "entering the year." It is the time of the "rainy season" in Thailand when monks are expected to remain at the temples. It generally marks the "start of the year" for Buddhist monks. Often some particularly devoted Theravada Buddhists will observe this time of year by restricting their diet to only certain vegetables.
But walk into most any Thai restaurant - either in Thailand, or somewhere in North America or Europe - and you are bound to be confronted by plenty of meat dishes, no matter what time of year or which particular religious festival is taking place.
On the other hand, it is important to remember that Southeast Asia does not have a monopoly on Buddhism and Buddhists. There are two main schools of Buddhism; Theravada, known as "Way of the Elders," which is common in southeast Asia, and Mahayana, known as "The Great Vehicle," which is the school found most often in India and China, Japan, Tibet, Korea, Nepal and Bhutan.
In the Mahayana school there are numerous different lineages, And each lineage might have their own rules concerning what their followers diet should be. So you are more likely to find Buddhist vegetarians who follow lineages in the Mahayana school of Buddhism. As an example, some of my favorite Chinese restaurants in America have a large section of their menus devoted to vegetarian food. One really popular plate is known as "Buddha's Delight," which is a plate of stir-fried vegetables, with fried tofu serving as a substitute for meat.
So in summary, no, you don't have to be a vegetarian to be a Buddhist in all schools and lineages of Buddhism, but there are certain lineages in the Mahayana school that DO expect you to be a vegetarian, and since the Mahayana school is more widely known in the West that the Theravada school, that has probably led to the confusion that ALL Buddhists are vegetarians.