In 1 Samuel 21, why did David and his men have to abstain from sexual relations to eat the holy bread?

Because ritual purity was required under the Mosaic law before approaching consecrated objects. Sexual activity, though not sinful, made a man ritually unclean for a time. David’s men could eat because they were ceremonially clean. The law taught that physical purity symbolized interior holiness, and this foreshadows the purity needed to receive the Eucharist.