In the ten commandments any sexual sin comes under the heading of adultery. If you are with anybody else besides your wife or future wife that God intended for you to be happy and fulfilled with it is considered adultery. It is also a part of the first two commandments which are to love God and your neighbor. If the sexual sin is causing you to be separated from God and is harming another person or using another person for your own personal gratification of lust rather than love then you are harming yourself and the other person.