A really important rule to go by

Mind your own business, unless people are being harmed. By harm, I mean serious physical or emotional harm, or anything done without another person's consent. This especially goes for stuff that should be personal (e.g. sex).

FAQ's:

Q: What if a person is hurting themselves?
A: You should definitely help them if they need it, but it really depends on exactly what they're doing. If they're self-harming, then you should definitely help them. If they're engaging in behaviors that have no physical effects, but could have negative emotional effects, then it's only your business if they tell you about it. For example, if a friend tells you that they're having casual sex, but they're doing it safely, then the only thing you can really do is talk to them about the negative effects.
If someone you care about is doing drugs, you should help them as much as possible. If a person is at risk of hurting others, you should definitely tell someone. But if we're talking about someone being thrown in jail for smoking pot, I'm totally against that.
The most important thing to keep in mind is that you should NEVER be judgmental. That just hurts the person even more.

Q: Does that mean abortion is okay?
A: Of course not. Abortion kills another human being.

Q: What if someone likes sadomasochist stuff? Doesn't that hurt another person? Even if it's consensual?
A: Well... if someone has a fetish that is really unsafe, then yes, it does hurt another person, and they should probably get psychological help (but that doesn't mean you should go around asking people if they do that stuff because that's just creepy). But if two consenting adults are doing something that causes some pain that won't cause any long-term damage, that's none of your business. If you don't like it, DON'T DO IT. Seriously, if you think that it's your business what consenting adults do in the bedroom, you're the real pervert, not them.

Q: What if they're doing something that God says is wrong?
A: First of all, Matthew 7:1 says "Judge not, that ye be not judged." It's okay to tell them what you believe, but I don't think they're going to be convinced to change if you tell them that they're going to burn in hell. Stand up for your beliefs, but be considerate of others.

Q: You don't have morals, you godless liberal.
A: False. If I didn't have morals, I would be okay with people hurting each other. And for the record, I believe in God, and I'm not a liberal (if anything, I have libertarian leanings).