Often referred to as fornication and is basically having sex without being married. From the position of Christianity this is largely considered to be wrong, though some Christians are not against it as long as there is a loving, committed relationship. Natural moral law is definitely against it. So too would Kant as marriage is more likely to be a duty and saw sex as part of this duty. Utilitarianism is flexible and will only consider whether it is the greatest good for the greatest number, though you will need to consider both Bentham and Mill’s approaches. Virtue ethics is more concerned with the extent to which such actions makes us virtuous. This is more likely to favour a committed relationship where the virtues of love and caring are present.