Robert Anson Heinlein
When any government, or any church for that matter, undertakes to say to its subjects, "This you may not read, this you must not see, this you are forbidden to know," the end result is tyranny and oppression, no matter how holy the motives. Mighty little force is needed to control a man whose mind has been hoodwinked; contrariwise, no amount of force can control a free man, a man whose mind is free. No, not the rack, not fission bombs, not anything you can't conquer a free man; the most you can do is kill him.
"If This Goes On", 1940
Take sex away from people. Make it forbidden, evil. Limit it to ritualistic breeding. Force it to back up into suppressed sadism. Then hand the people a scapegoat to hate. Let them kill a scapegoat occasionally for cathartic release. The mechanism is ages old. Tyrants used it centuries before the word "psychology" was ever invented. It works, too.
"Revolt in 2100", 1953
Love is the condition in which the happiness of another person is essential to your own.
"Stranger in a Strange Land", 1961
Masturbation is cheap, clean, convenient, and free of any possibility of wrongdoing and you don't have to go home in the cold. But it's lonely.
"Time Enough for Love", 1973
Sin lies only in hurting others unnecessarily. All other "sins" are invented nonsense. (Hurting yourself is not sinful just stupid.)
Ibid.
The more you love, the more you can love and the more intensely you love. Nor is there any limit on how many you can love. If a person had time enough, he could love all of that majority who are decent and just.
Ibid.
There is much more but,