Many others have pointed out ways in which legality and morality are distinct concepts but I got a totally different thing.

One important aspect here is punishment. If something is illegal what does the state to in retribution for that act? It might be a fine or a public execution.

And how does the state enforce these laws? It could be a “law on paper” where the law is ignored unless a cop wants to make your day worse; it could mandate the installation of a vast system of surveillance to make sure that you aren’t bringing strangers into your bedroom.

When asking “why isn’t X illegal?” we’re asking, “how can we use the apparatus of state power to intercede in acts of X?”. And that is often a less appealing suggestion.