1. It is a blood covenant
Since there is exchange of blood through the genital organs of those who have sex, it is invariably a blood covenant.
2. It leaks your secret
The genital organ of a man(male & female) is a secret that God entrusts to him to share it with the opposite sex for a divine purpose (procreation). Once you leak the secret to a stranger who is not ordained by God, it degrades your integrity in the sight of God,
3. It changes you into a monster
In the spiritual life, couples who are joined in marriage are one. (1+1=1). Their oneness or unity is of God, so they are one in the beauty of God but in premarital sex, the two people will become one but since their unity is of the devil, they will be like a monster (a man with two heads, four arms, four legs but one body). It's the reality of the spiritual life. Only the blood of Jesus can separate your spirit from the other.