This is such a serious thing that the Mosaic Law placed the penalty of death on adultery and other acts of sexual perversion was death. It is true that especially in the Old Testament this law was often ignored and violated. Polygamy was often practiced. But polygamy is never shown in a good light or presented as God's ideal. Sex outside of marriage is not condemned because sex as such is ugly, bad or dirty, but rather because sex is beautiful, good, and pure.
Sex is powerful both in the desires it arouses and in the results it produces.
Some have tried to tell us that sex is just a physical act like shaking hands. This is of course sheer foolishness. Shaking hands doesn't stir ones deepest emotional cords, a person doesn't get AIDS from shaking hands, and shaking hand certainly does not bring new life into the world. It brings guilt, shame, and reproach, feeling which play havoc with ones emotional and mental well being. It is not unusual for adultery to result in physical decease.