Extra marital sex
Is adultery. Christianity is against it though you would also need to produce a faith and reason argument against it is considering natural moral law. The fact that it is one of the 10 commandments certainly helps. It would be seen as a misuse of sex. Kant would not be able to universalise it as it would require everyone to do it which makes marriage pointless in the first place. Utilitarianism is going to consider the utility principle and obviously if harm is caused it would not accept it. However you may consider if it is guaranteed that harm would be caused. How certain can we be that the partner cheated on would find out. Remember utilitarianism never says anything is absolutely wrong. However don’t forget the different types of utilitarianism. It is difficult to see how adultery could be regarded as leading to virtue, especially as loyalty and honesty are considered to be virtues.