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I also think that because of the fact that sex is also creating a spiritual bond between people having sex(Gen2:24 and they shall be one flesh) the sins of the persons having sex are transferred to one another,as well as the iniquities of their forefathers which were not confessed,thus making them impure
Wow, Rudolph.... That's an amazing insight. I did hear an STD medic say that too, in that "When you have sex with one person you are sleeping with everyone else they have been with."
Sexual purity strengthens the marriage bond and thereby helps parents raise well balanced confident children, the basis for a strong society.