Marriage represents a picture of Jesus' love for the church (us). I think the marriages that work are because before that, the couple spent a season alone...experiencing a relationship with Jesus first. Until you have your love life with Jesus figured out and you find a guy/girl who has their love life with Jesus figured out, you cannot paint a picture of the gospel. You have to love him (Christ) first and trust him with your love life before you get involved with anyone.
In Genesis 4:1, "Adam lay with Eve and she became pregnant..." Lay in Hebrew means Yada. Yada means to know, to be known, or to be respected. God created sex so that you could bond to someone. Your partner will know you in a way that no one else does. Kind of how God knows us in a way that no one else does.
That's just my 2 cents. So in a way I can see how the users who said "marriage is just for sex" could be truthful. But you have to see sex in a very different way than the world does. It's not just for physical pleasure...it's for bonding and knowing someone similarly to how God knows us.
Of course I've never been married or been in love...so take this with a grain of salt. I just reread my notes on the subject.
But I think Dannah Gresh knows what she's talking about.