“What does it mean that ‘the sexually immoral person sins against his own body’?”
The quotation here comes from a longer discourse by the apostle Paul on the importance of Christian doctrine and practice in the area of sexuality. It includes the memorable line “Shall I then take the members of Christ and make them members of a prostitute?” — the obvious answer being “Never!” The relevant portion is as follows: “Flee from sexual immorality. Every other sin a person commits is outside the body, but the sexually immoral person sins against his own body.”
So how does that work exactly?