1. For the first time in human history, it's a viable thing to live alone. Not only that, but actually living alone is easier than living with a spouse. If you want to be married you have to put a lot of effort into dealing with the other person. If you're single, nothing stops you from just eating frozen pizza and playing video games all you want. 100 years ago most people were farmers, and good luck trying to farm land alone, especially as a woman. Once the economic survival necessity to marry disappeared, the naked truth was exposed - people just don't like each other. We'd rather sit alone.
2. When people were finally taught "if you put penis into vagina without latex, a child will pop out, and you will need to keep feeding it for next twenty years", the reaction was "fuck that" because honestly, parenthood is just a bad deal. Nature got around this problem by equipping us with sex drive and for most animals that works because most animals don't make the mental connection between sex and pregnancy, but modern people do make the connection, which breaks the system.
3. Demographic collapse is actually not a problem. Humanity will not die out. It's just a giant evolutionary bottleneck, like many in history. Will the society collapse? Maybe. Not the first and not the last time.
4. Even if the society collapses, a better one will emerge. Black Death killed half of Europe's population, and some historians say that this contributed to Renaissanace, which was a major postive turning point. Labor shortages led to much better position of common workers, and the last time I checked, common workers bitched about being exploited.