There's a popular song in Christian music right now by Casting Crowns called Broken Together. A line in the song refers to marriage as "hallowed ground". Every time I hear this song, and that line specifically, God takes my mind to my marriage and the mental images He gives me of what it means that marriage really is a holy place.
I don't know that the Bible ever specifically mentions marriage being holy. I can't point to a specific verse that says, "Hey, remember your marriage is a holy thing - treat it that way." But I think the way the Bible defines and describes marriage definitely points us in that direction.
Let me flesh this out a bit...
God is a holy God. Holy meaning set apart, perfect. God is holy, perfect, sinless. And our holy, perfect God set up this great thing called marriage in which a man and a woman mirror God's relationship with His church. They commit to each other for life, as God commits to His church for eternity. Since we humans fell into sin, marriage is no longer the perfect union it was set up to be back in Eden. But, it's still that picture, still that image, albeit broken, of God's relationship with us. It's still a holy thing - marriage is still meant to be set apart for God's purposes. And I think if the Apostle Paul was living today he probably would say something like, "Hey, remember your marriage is a holy thing - treat it that way." In fact, Ephesians 5 talks about this very thing.