I just spent the past ten days in Amsterdam and it only took eight for me to notice the XXX symbol plastered everywhere, so not that bad on the observation front. First on Tinder and then on a massive advertisement over a canal, I could not figure out the connection. Initially, I'm sure most people would assume that the XXX symbol has something to do with the city's infamous adult entertainment industry. However, it is merely coincidence that the city's historic symbol has also become a symbol for the sex industry.
The XXX emblem, which appears on the city's flag and coat of arms, is made up of three St. Andrew's Crosses in white on a black stripe surrounded by a red background. St. Andrew was a fisherman who was crucified on an X-shaped cross in the 1st century AD. In 1505, the fisherman's town made it their official symbol and it flew on the flags of all ships registered in Amsterdam.
Along with wooden clogs, tulips and marijuana leaves, the city flag is one of the more common things you will find represented at souvenir shops. This, in addition to its funny accidental connection to Amsterdam's sex tourism, certainly helps sell more souvenirs. Even so, the city is proud of the symbol and all of its meanings.