Is it right or wrong, and why?
In terms of ethics. You may wish to pin down what ethics they are talking about - personal ethics, or social ethics. In social ethics, it doesn't have to be the whole of societies ethics, but the group they belong to's ethics (like a church doesn't have exactly the same ethics as the rest of society).
In terms of laws, you may wish to discuss how they government doesn't intrude into all aspects of life, for a reason - showing its legality.
You could discuss how morals came into existence and why (ie. why the monogamous marriage) - to use as a comparison later.
You could discuss the spread of disease. The percentage practice of safe sex etc.
Also you can discuss the difference between love & lust. And the acceptability (or not) of just plain lust.
You could also discuss the types of abuse that people engaged in it may attract (guys being admired, girls being called sluts), and the affect it has on them.
And from one side, you may wish to discuss the concept of knowledge, and consent. You can reference this back to your discussion on laws, and ethics.
From the other side you may wish to discuss the emotional issues that often arise (consenting because of lust, but in love, and the issues that arise. Or when one partner starts developing deeper feelings while the other doesn't, etc)
In the end, we humans are emotional creatures, so this last part can never be underestimated in its affect on our lives.
You are of course, discussing a subjective topic.