Just a few days ago, I finished reading an autobiography called Bangkok Boy, written by Chai Pinit. I believe I will tell you why there are so many mixed-race people in Bangkok.
What I want to pave the way for you here is that there are many Chinese in Thailand, including Thaksin Shinawatra and Yingla, whom we are familiar with, and there are also many native Thais. When they get married, they can be regarded as hybrids. The familiar Thai star Mario Maurer is a Sino-German hybrid. These hybrids born through normal marriage are actually everywhere.
As we all know, Bangkok is famous for its tourism and pornography. Many girls come out to stand in the street, while Asian girls tend to be western (or even black) men. Many people do not pay attention to safety measures, so many mixed-race children are born. What's more worth mentioning is that after these children are born, their fathers may return to the west, and their mothers may transfer them to Thai families with better conditions for their livelihood. This situation is described in detail in the book Bangkok Boy. If you want to know more, you can read this book.