Logic, reason, understanding, empathy.
There is no "Atheist" answer to morality. Lacking a believe in a deity doesn't decide where your morals come from, it just means that they don't
come from a deity. The above is my personal answer.
I usually follow a similar idea to the Safe, Sane and Consensual plan.
First, a person must consent if anything negative would happen to them(don't have to consent to surprise presents or whatever, though annoyance is a
negative if you do it bad.)
A person must be able to consent(Knowledgeable about the consequences, mentally able to register them and decide properly.
Generally, if all affected parties can consent to what they are doing, and do so, it's moral. Parties not affected at all, don't need to
consent(Such as a gay couple not needing Westboros consent for them to be together).
However, that's morality. Dumb actions is a whole other can of worms. Such as risking addictions to heavy drugs or consenting to that damage to your
body. Pointlessly risking STDs by being excessively promiscuous. Ect. Just because something can be done within the frames of morality, doesn't
necessarily stop it from being a bad action.
Social Conditioning is really strong though. Grow up always told that sex is immoral when both sides consent but one side is doing it for cash, and
you'll have a hard time shaking that feeling. Same with premarital sex or open relations. Same with eating Pork, or Shellfish, Getting Divorces,
This is why a generally draw a line between sin and morality. Despite some overlaps, a sins a sin and an immoral action is an immoral action. There
are no sins in my believe, so I only go by morality. A religious person should go by their beliefs in sin, but should not cease to care about morality