It is a God's gift to His creations for survival.It should beyed in a self controoled manner.As you know too much of everything is bad sex is also bad when it is not controled.You know liquor is injurious to health when it is taken too much and if used in the manner and quantity it is medicine.Medicines in allopathy and homeo,ayurveda etc. contains a certain limit of alchahol.So sex is an inevitable part of healthy humanity long life which makes life beautyful.