Every Thai has to become a monk once before marriage. Can it be long or short?

No, not every Thai has to become a monk once before getting married. Although Thailand is a Buddhist country, it also has non-Buddhist beliefs. Such as Islam and Christianity. They wouldn't have to become monks.

Even Buddhist families don't have to become monks once to get married. Because Thai Buddhists believe that short-term becoming a monk before marriage is to repay parents and increase their happiness. A short-term monk after marriage is for his wife, and people will laugh at him. Moreover, people who have never been a monk before marriage have to work hard to get a wife. Therefore, most men will have at least one short-term monk experience during their adolescence or before marriage. But not all Buddhists do this. This is illegal.