I think one reason young couples are surprised by the realities of "marriage" is that so much is said about "weddings"; everyone's all excited about the one big day; the "marriage" that comes after might not be mentioned at all until reality sets in.
I think that it's a good thing that nowadays, it's more usual for couples to live together before marrying, giving them an insight into the reality of being a couple. I find it hard to imagine the idea of only finding out the reality of living together after the wedding, which was once the norm.
When reading wedding magazines, I noted with disgust that they had plenty to say about dresses, confetti, ceremonies, limousines, toastmasters, but barely a word about marriage. (Yes, I know, they have an agenda; they're not going to put people off wedding by telling them about the mundane reality of marriage.) We anticipated that we needed something to occupy ourselves once the big day and the honeymoon had come and gone. Seven years later (and we heard a lot of teases about the "seven year itch"), we have to work to keep our marriage happy and interesting, find new excitements; in a sense that is hard work; and also the realities of life, as others have said, which change as everyone gets older.