The Gemara at the end of amud beis discusses the preparation required in order to enter in the Holy of Holies. The principal requirement is six days of separation, however ultimately it is mandated at seven days. 

I heard a lovely idea from Rabbi Avrohom Blumenkrantz ZT”L. The seven days of preparation that the Cohen engages in to prepare for entry into the Holy of Holies is paralleled by the seven days of preparation that a couple have prior to immersion in the Mikvah, and resuming sexuality. He said, the bedroom is like the Holy of Holies and requires the same preparation.

Holiness and the idea of having something sacred is a universal concept that spans all cultures and religions. How do you define holiness?  Rudolph Otto, in his treatise, the Idea of the Holy, defines holy as, “As an experience or feeling which is not based on reason or sensory stimulation and represents the wholly other." Meaning, there is a sense of something, but it is not coming from your senses yet you still know it’s there. Something outside yourself that is not coming in from sense-data reality. To encounter this otherness certainly does require preparation in order to become more attuned.

What happens in the bedroom when people become sexually intimate and value it as such, is certainly holy. It is the encounter of the other person's soul, something you cannot feel and yet know is there. Indeed, make adequate preparation.