One of the downsides of living in modern day western society is trying to shrug off the outdated leftovers of our ancestors' ancient religions. Even today, after the major sexual revolutions of the last century, the benchmark for "morality" doesn't go much further than extramarital sex. Teenagers talking about getting to "first base", and even young adults discuss "how far they've gone" with someone as a way of defining social standing, and for many people, the novelty of fucking yet another person doesn't wear off, as the sheer breadth of euphemisms surrounding this suggests.
But this is immature. It's the twenty first century, and yet the general concensus on sex after "you should wait til marriage" doesn't really go much further than "if he cheats on you then dump his ass", which is more of a knee-jerk emotive reaction than anything else, although it does provide us with a starting point.
Before we get into details, let's ask the first question - why do we need ethics for sex? Because sex is powerful. It can bring great joy in itself, can grow and cement relationships, but can also hurt people deeply. With power comes responsibility, and yet so many people who choose to be promiscuous, either in the short term before "settling down", or on the long term as a lifestyle, see it as an opportunity to shed responsibility for a few nights every week, before returning to the rat race. Yet sex is an area where responsibility is sometimes even more important than in day-to-day life. At work, errors in judgement can cost money, but with sex, errors come with costs to both emotional and physical health. A few hundred dollars misplaced is temporary, but an STI or broken trust can last for much longer. Hence the urgent need for responsibility, and therefore, the need for ethics.
So where do we start when defining sexual ethics? The simplest way to do this is to look at the effects of sex, which we can categorise as good, bad, and neutral. The purpose of sexual ethics then, is to maximise the good effects, while minimising and mitigating against negative ones. Seems simple enough, right? Let's start on ethics for good then.
1) Use sex for good (be excellent to each other)
This is more useful than it seems at first glance. I've heard lots of (moral? ethical?) discussions around whether or not sex is okay in X circumstance (theoretical, but also "im meeting x this weekend and..."), and they usually end in a contrast between the persons desires, and what will happen to his/her social status as a result. The topics vary, from taking someone's virginity, to how drunk people can be, to what constitutes coercion (and therefore rape), to more mundane topics, such as how you shouldn't fuck your friend's sister, or how much detail you tell people after the act.
All of these topics can be easily addressed with the question "am I doing this for good?" It's a nice and simple question, but it allows for proper personal critique. It leads to a few follow-on questions - "am I doing this for me? for him/her? are we doing this for our mutual enjoyment?", and the answers to these questions give you an idea of your intentions.
If your intentions are good, then you're off to a good start. But people can be still get hurt despite the best intentions, and this leads us to our next ethic
2) Do no harm (don't be a dickhead)
Also known as the hippocratic oath, the focus of this is on mitigating harm. This is simply the complement to the first ethic, but this time the focus is different. The purpose of this ethic is to be aware of any negative influences driving our sex, from obligation and coercion, to simply using other people for our own personal gain. This is where cheating comes in - you can have sex with someone for all the right reasons (thus following ethic 1), but be betraying your commitment to someone else, and therefore indirectly hurting someone else. In the same way, using sex to get back at someone else is also bad, as not only are you using your current partner as a means to an end, but you are also aiming to hurt a third person. This sort of behaviour is destructive, and unethical.
The question to ask yourself here is "am I hurting someone?" Sometimes people use others for sex in order to feel better about themselves, either directly, or through later bragging about their "conquests". Here the end goal may be personal gain, but it's not healthy, and at the end of the day the sex is simply a means to an end, rather than a wholesome end in itself. Sometimes, despite our best efforts, we can still hurt other people, but by looking to find ways in which we might be causing harm, we become aware of possible side-effects, and are then more likely to avoid them.
These two ethics form the basis of our code, which culminates in the final overarching key to sexual responsibility:
3) Take ownership (if you can't love yourself, who can you love)
An impotant part of taking responsibility for our actions is to ensure that they are congruent with our own definition of ourselves. The purpose of the first two ethics is to ensure the safety and wellbeing of the other people involved (and to an extent, to avoid ourselves engaging in self-destructive behaviour), but the purpose of the third ethic is to ensure we aren't pretending to be something we aren't, or being ashamed of who we truly are. To take ownership of your actions is to identify with them, and to be proud of yourself for it. If you conduct yourself ethically, then there is no need to be ashamed, and treat sex as something dirty or smutty, but instead to see yourself as a healthy, sexual individual. The stigma of sex as selfish and hedonistic falls away, as do superficial benchmarks such as the number of people you fuck and how sexy everyone else thinks they are.
By taking ownership of your sex, you find that who you fuck is your business, and if you conduct yourself ethically, then any arbitrary "morals" people try to impose on you are simply meaningless. Your intentions speak for themselves.