First Corinthians 6:18 “Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body.”
Paul gives us good advice when it comes to sexual immorality. Run! Get away from this type of relationship as fast as your feet can carry you.
All sin is evil but sinning against your own body is worse because this sin is against the body while other sins are “outside of [the] body.”
Paul warns that “the unrighteous will not inherit the kingdom of God” (1 Cor 6:9) and if we are joined with another person in sexual fornication outside of marriage we are actually “joined to a harlot’s body” (1 Cor 6:16) and the two “become one flesh” (1 Cor 6:16).