It was not holy, it was sensual indulgence- which is lack of self-control- which is one of the fruits of the Spirit. It was hard-hearted-ness. David was a man after God's own heart for the most part, but he sinned just like everyone else. Yes it was a sin, we are supposed to love God and everyone. How was it love to his wife to get other wives?
Jesus said Moses permitted you to divorce because your hearts were hardened, but it was not this way from the beginning. God made one wife for Adam, and one husband for Eve. And in the qualifications for elders, which actually are qualities every Christian should have but elders must have, is that they must be the husband of one wife. Jesus has one bride, to have more is not a loving nor holy thing to do.