You are assuming that all moral codes come from religion. Some moral codes have just simply evolved over the millenia as people observe what works for society and what doesn't.
Behaviors that inherently harm other people are obviously immoral. Why do you need a book to tell you that?
If I am alive, I know what that is like and that it allows me to do things and enjoy things. When I see a dead person, I see that he can't do those things any more. If he is dead because of natural causes, that is sad. If he is dead because someone took life away from him, that is wrong.
Everyone assumes that atheists are immoral because we don't get our moral codes out of a religious book. Behaviors that always harm people are bad. Why is that complicated?
Atheists and religious people agree that these behaviors are bad, though we disagree on the source of the moral code.
The other category of moral codes are those that don't harm another person.
These include booze and almost anything related to sex. Except adultery, which is a form of cheating and hurts another person.
You claim that other people are harmed by them, but the harm is often greatly exaggerated.
People often speak out against porn in general, but when you look at some of the arguments they make, it turns out they are really talking about kiddy porn, which is a radically different thing practiced by a few people living deep underground. Arguments related to kiddy porn have nothing to do with regular porn, and the two are not used by the same people.
People make claims that women have been forced into porn, but for every Linda Lovelace or Traci Lords that claims that, there is a Ginger Lynn or a Jewell D'Nyle who proclaims she loves being a porn star, and that no one made her do anything. My personal opinion is that Linda and Traci were overcome with guilt later and manufactured someone else to put the blame on. Linda is known to have been born again since her porn days. Hence the sudden guilt.
Alcoholism and drunk driving are always used as an argument against booze, despite the many people who drink booze without problems.
Teen preganancy and STDs are used to decry unmarried sex, despite the millions of adults and teens who have sex with no problems because they protect themselves.
The behaviors we are disputing can sometimes hurt people, expecially in combination with other bad behaviors (such as not using protection).
The behaviors we do agree on always hurt people.
Killing someone, cheating someone, lying to someone, defrauding someone, assaulting someone is always harmful.
Unmarried sex, booze, porn, etc., can sometimes be harmful, depending on circumstances. But more often than not, it isn't harmful.
This is the difference between the laws we support and the ones we feel you force on us.