Why should the law care about what I do behind closed doors?

The law doesn't care about what you do behind closed doors as long as it is within the bounds of what the law allows.

You can stand on a public sidewalk and take pictures of my house. You can't walk into my house uninvited and start taking pictures.

Society sets laws announcing those actions that it deems unacceptable in polite society. If evidence appears that causes a reasonable person to suspect that illegal activity is going on, society should investigate. Of course society might find itself having to jump through hoops by adhering to constitutional law. It cannot just invade your personal space and demand to know what you're up to just because they don't like you.

2-million year-old pitted teeth from our ancient relatives reveal secrets about human evolution - The Conversation

If evidence arises that you are doing these things behind closed doors, don't you think the government has a moral obligation to investigate?

It shouldn't to a point.

But what if you're raping little girls behind closed doors? Killing gay men? Watching child porn?

Whenever I write a novel, I struggle with the end, should I make it open? Should the good win or the bad win? Sometime I don't even have an ending, what should I do?

Liberty is not boundless. It does have its limits.

Your speech is free. But if it causes malicious harm to someone, you can be sued.

The law shouldn't care (if you are a law abiding citizen) about who you take into the bedroom as long as they are consenting adults. How many guns you own. What you eat for supper. What kind of TV shows you watch. Whether you watch porn or not.

McDonald’s bringing it’s own version of iconic dessert to its McFlurry menu - MLive.com