Can sex be spiritual? Yes, but we must know exactly what it means, otherwise, there will be havoc. Srila Prabhupada explains in details here how to dovetail sex impulse in Krsna consciousness by begetting Krsna conscious children.
"The heavenly pleasure for the conditioned soul is sexual pleasure, and this pleasure is tasted by the genitals. The woman is the object of sexual pleasure, and both the sense perception of sexual pleasure and the woman are controlled by the Prajapati, who is under the control of the Lord's genitals. The impersonalist must know from this verse that the Lord is not impersonal, for He has His genitals, on which all the pleasurable objects of sex depend. No one would have taken the trouble to maintain children if there were no taste of heavenly nectar by means of sexual intercourse.
This material world is created to give the conditioned souls a chance for rejuvenation for going back home, back to Godhead, and therefore generation of the living being is necessary for upkeep of the purpose of creation. Sexual pleasure is an impetus for such action, and as such one can even serve the Lord in the act of such sexual pleasure. The service is counted when the children born of such sexual pleasure are properly trained in God consciousness.
The whole idea of material creation is to revive the dormant God consciousness of the living entity. In forms of life other than the human form, sexual pleasure is prominent without any motive of service for the mission of the Lord. But in the human form of life the conditioned soul can render service to the Lord by creating progeny suitable for the attainment of salvation. One can beget hundreds of children and enjoy the celestial pleasure of sexual intercourse, provided he is able to train the children in God consciousness.
Otherwise begetting children is on the level of the swine. Rather, the swine is more expert than the human being because the swine can beget a dozen piglets at a time, whereas the human being can give birth to only one at a time. So one should always remember that the genitals, sexual pleasure, the woman and the offspring are all related in the service of the Lord, and one who forgets this relationship in the service of the Supreme Lord becomes subjected to the threefold miseries of material existence by the laws of nature. Perception of sexual pleasure is there even in the body of the dog, but there is no sense of God consciousness. The human form of life is distinct from that of the dog by the perception of God consciousness.
SB 2.10.26 purport