There is no such thing as premarital sex or sex before marriage. To have sex with any person is to marry that person.
This idea strengthens the notion of common-law marriage in which cohabitation is deemed a legal marriage in some parts. It is also the reason why some cultures fail to recognize a marriage as valid until it has been consummated via sexual intercourse.
Simply put, sex equals marriage. Even the good book says that to sleep with anyone is to become one flesh with them.
Now think about all the harm that fornication and adultery do in this world. If God created the sexual union called marriage to be manifestation of His image, imagine how He must feel each time His child indulges in sexual sin.
The good book says, God created man in His image, male and female. And for this reason, a man shall leave father and mother and cleave to his wife and the two shall become one flesh.
When a person cheats on a spouse, it isn’t just a stepping out of marriage for pleasure , revenge or whatever excuse there might be; it is the creation of a monstrosity that mocks the image of God.