If it causes you harm, since you don't live in a vacuum or alone in Antarctica with the penguins, it will also harm others. If I harm myself by taking hard drugs, in private, that will harm my family, my neighbors, and my community (in public). If the government allows incest, polygamy, polyandry, sex in public, pedophilia (there are "woke" leftists arguing for legalizing pedo-sex), pornography, and also gay marriage, all of that undermines public health, safety, and order. The government should regulate it because, at the end of the day, when people foul up, it's often the government (the public, society), that has to pick up the tab and "bailout" the miscreants and screw-ups.