(Kadamba Kanana Swami, November 2015, Vrindavan, India, Video Interview)
The essence is to consider the goal of life. When we think that sex is the goal of life, then sex is bad because then sex will be engaged in only on the basis of lust and day-by-day, we will become more and more bound up in material existence and in the suffering that the material world brings.
But when we understand that Krsna is the goal of life and we engage in sexual activities according to his direction, then we are engaging according to the injunctions of the Bhagavad-gita, in accordance with religious principles, as it is authorized by Krsna. Such sexual activity is very nice and purifying. So to say that sex is bad would not be true.