Hook ups become unethical when someone is cheating on a partner who does not know about the hook ups; or when someone is coerced into the activity after they've shown reluctance; or when someone is too young to fully understand or appreciate what's involved. (irrespective of age, someone can be much more immature than others in the group).
It's unethical to hook up when you're in a relationship outside, or when you can see the possible lay is really keen on you. This will lead to hurt when you move on to the next sexual encounter.
