What is the best and most correct way to wash clothes after sexual intercourse, is it with water only or with water and soap?

A garment does not become impure merely by being worn during intercourse unless an impurity gets on it, and semen is not impure. If an impurity gets on the garment, only the affected spot should be washed if it is known. If its location is uncertain, the entire garment should be washed with water, and that is sufficient without the need for soap.