Marriage system in Thailand

Traditionally, there are many forms of marriage system in Thailand, such as group marriage, pairing marriage and monogamy. In social practice, we can often see some marriage customs derived from the above marriage system. If you live from your wife, that is, the man will go to the woman's house for a long time; Service marriage, that is, the man works in the woman's house for 2 ~ 3 years and can only get married after taking the exam. In some places, there are also wedding robberies. Cohabitation before marriage (trial marriage), polygamy, etc. According to the current laws in Thailand, it is monogamy, but traditionally, the king and the people can be polygamous and advocate it. It is normal to have three or four wives, but only one wife is legally recognized, but children born out of wedlock are allowed to take their father's surname and have the right to inherit. In other words, the law clearly protects the results of extramarital affairs. Isn't it contradictory?