God made marriage to be Holy, when he first created them man and woman. after the Incarnation the Lord began his first ministry with the blessing of the marriage in Canna.
sex within marriage is a beautiful sanctified act of complete and perfect love, the giving of self and the accepting of another with reverence, joy,and love. in such holy union the creator himself is present in that joining that goes deeper than the flesh, this union is protected and sanctified by Him, that which the lord has joined let no man divide, that includes those two involved, selfishness is the antithesis of genuine love, denying the other person of sex without due reason ( factual health issues/as some have been known to fabricate them/, agreed upon fasting time , agreed upon reason for abstaining /whatever that may be/ that's all i can think of right now)is a sin against the person, thus against the marriage and it will open a door for the Evil One to enter.
from holy matrimony and within its holy union men are made perfect/ saints ( the married couple), and if God wills it, saints are born in the co-creation that is possible in this union. how sacred is this union! where I come from, under normal circumstances people do not sit on married couples bed, because it is seen as a very holy place of holy act,reserved only for them. in the country side, it is a part of genuine hospitality for them to receive a guest , wash the feet of the guest and as a sign of greatly honoring the guest,leave their bed for the guest to sleep on and they sleep on either the floor or on another bed.