Morality is a personal preference. It's a matter of taste. Unfortunately, religions have co-opted it and used it as a tool for controlling the masses. They tell their followers what morals they should hold, and then they try to pull the rest of society into line with those morals - because after all, each religion has the right answer, and every other religion is wrong.
What is morality but a guide for how to live a pure life free from sin? But who decides what is sin? Is sin a scientific concept, dictating ways of life that are harmful to the individual or society? Or is it an arbitrary designation made by uninformed zealots who once perceived this or that way of life to be harmful (or merely distasteful) in one way or another?
There are two kinds of immoral acts. One is a sin against others (e.g., crimes like theft and murder), and the other is a sin against oneself (e.g., vices such as gambling and promiscuity). But I can argue that with the philosophy of ethics and the principles of science, the concept of morality is completely irrelevant and extraneous to a complete guide for living well.
In the first case, we have sins against others, which can be completely covered by the rule of law, correctly applied - that is, applied for the protection of individual rights, and not as a moral prescription for living a clean life. When one person 'transgresses' against another, in the sense of violating the other's rights (such as in stealing from him, or killing him), we have a clear instance of crime that can be investigated, judged, and - if proven - punished. There is clearly a transgressor, and there is clearly a victim, and the execution of the crime clearly threatens the sanctity of the social construct. Ethics, as applied to dealings between individuals with concern for those individuals' equal rights, is all that is necessary to govern sins committed by one against another, and is the full proper application of the law.
In the second case, we have sins against oneself, which are commonly viewed as vices. While it is noble to have concern for one's fellow man, and to have a desire to protect him from harm and lead him to a pure life, there is a broad margin for interpretation when it comes to deciding what type of life is 'good', and whether or not 'purity' is even something to be valued. Ultimately, concern for the rights of the individual, and his ability to freely decide how to live his own life, trumps any moral concern for the decisions he chooses to make in life. It is perfectly fine to advise people on the rewards and pitfalls of various lifestyles, but ultimately, the decision lies with the individual, and that means we must be willing to respect individuals who choose to live in ways we may not support or recommend.
Now, regarding how we determine which lifestyles are 'good' and which are 'bad', we don't need a fictional being to dictate to us what those are. We are not children. We have developed the field of science as a means of understanding the world around us. Through objective scientific trials, we can determine which ways of life are harmful and which are beneficial, and we can observe precisely in what way(s) they are harmful or beneficial. And if science provides no evidence of harm, then there is no reason to discourage that lifestyle. We need not say that promiscuity is a vice because God forbids it, or because the Devil encourages it. Rather, we can say that promiscuity carries the risk of transfer of disease, unwanted pregnancies, and the like.
And, science also gives us the tools to isolate those risks and work on preventive measures so that a person can live promiscuously with less risk than they could before. This is the power of man - shaping the world for our own betterment. And in the end, risk or no, it's up to an individual whether he wants to engage in a vice or not, and we should respect that decision even as we disagree with it. For there is no value in choosing to live well if we cannot also choose to live poorly (which is, after all, a subjective value judgment that one who lives in vice may fully disagree with).
Because it is so subjective, morality is a thing that must be applied only to oneself, and not to others, if we are to uphold the virtue of individual rights and free choice. We can make suggestions for how others should live their life, but in the end it is their choice. Acts that infringe on the rights of others can be prosecuted by law, and lifestyles that carry scientifically proven risk of harm to oneself may be discouraged (and those risks should certainly be targeted by education), but must ultimately be permitted to be engaged in by those who choose to do so. After all, life itself is not without risk, and a person should be free to weigh the risks he takes with the benefits they bring him - this is something he alone can decide, for he alone knows what the value of those risks and benefits are to him personally.
And when it comes to activities that are scientifically safe and criminally benign - like, say, dropping a quarter at the slots, or sharing taboo sexual fantasies, or anything that has been deemed unhealthy without proof, or offensive without demonstrated empirical harm - morality dictates only whether or not you like it, and it is not fair for you to discriminate against others on that basis alone. Morals ought to guide your own life - noone else's.