Great post from Dave Harvey at Gospel Coalition . . . 
Kimm and I had one premarital session before our wedding. It lasted maybe five minutes, just long enough for the well-meaning counselor to hand us a crate of cassettes and urge us to listen. We threw them in the trunk. One day, nine months later, he wanted them back. Not a problem, since they were right where I left them—in the trunk, unopened and unused.
It’s frightening to think how unprepared we were for marriage. I don’t blame our counselor. I’m not sure he had premarital counseling either. But as I reflect back on the last 35 years, there have been a few surprises it would have been helpful to know about.
Here are six surprises I believe every pastor or premarital counselor should cover:

1. The Sin Surprise 

Engagement is like walking through an amusement park with fogged-up glasses. There’s so much you don’t see clearly, but who really cares? You’re having fun! Here’s the truth: Your fiancé is more sinful than you know. If their sin hasn’t already surprised you, get ready: it will. I’m not saying your future spouse is hiding something. You just don’t have eyes to see what’s there. This is why you should seek counsel from friends, family, and the church before a relationship gets serious.
Borrow others’ glasses to look at your loved one through their eyes. Also, be sure to talk about the “three P’s” of past sin—patterns, partners, and particulars. Don’t be unnerved by what you uncover. Your beloved is a sinner just like you. Remember: Our sin is horrific enough to require Christ’s blood to take it away. But God’s grace has power over the “sin eruptions” you couldn’t see before your wedding day. Don’t be afraid. The fallenness you uncover becomes a theater for displaying Christ’s redemption.

2. The Conflict Surprise

I thought the early years of marriage were about how Kimm needed to improve. You can guess where that led. According to marriage gurus, our early conflicts simply indicated a lack of communication skill. But the Bible says, “What causes quarrels and fights among you? Is it not your passions at war within you?” (James 4:1–2).
Fights and arguments happen when we don’t get what we desire. My early conflicts with Kimm revealed what I craved. I got angry with her, because, well, I had an entitled heart. I wanted Kimm to respect me. I thought being respected was some kind of inalienable right grounded in both Scripture and the American constitution. But it didn’t take long before I saw how a good desire can corrupt into a harmful demand.
I thought each biblical command for my wife revealed a need in me and a right I possessed. But I came to see this takes God out of the picture—and puts me in his place. Sure, a respectful wife contributes to marital harmony. But God’s commands for Kimm exist to help her grow in love for him. They weren’t given for me to manipulate to my own ends.

3. The ‘Slow-Change’ Surprise

Walk in a dark room and throw on the switch. What happens? The room instantly transforms. We want spiritual change the same way: Hear a passage, throw on the switch of application, and change comes within the hour. That would make sense if Christianity were a vending machine. Put in your quarters and wait for the sanctification soda.
But God orders the pace of change according to factors we can’t see. Sometimes he gives it slowly to humble us. This reminds us we aren’t him. Sometimes he gives change slowly to tutor our spouse in patience, love, and mercy. When two people are yoked together, God’s growth of one always has the other’s soul in view. Demanding immediate change in a new spouse is a great way to introduce other problems into the marriage.
Since change takes time, we must help young couples cultivate confidence in the good news, lest they be tempted to grow weary or angry. The gospel has appeared, and it teaches us to live upright and godly lives while we wait for Christ’s appearing (Titus 2:11–13). The change Christ will bring is worth the wait.

4. The Sex Surprise

Here’s the sex surprise. You get married with a Disney mindset. You expect it’ll all happen perfectly and you’ll live happily ever after. But sex is unpredictable. Some discover their bodies were made to be intertwined, and the honeymoon begins a life of sexual adventure. They’re surprised it works so well; it was meant to be. But for many, sex is far harder than they imagined—whether it’s the past, physical pain, inhibitions and shame, difficulty finding a rhythm, or the cloud of sexual abuse.
You’re surprised the marriage bed requires so much assembly—so much commitment and work. For many Christians, sex is “meh.” In the first century, Paul had to talk to the Corinthian church about sexual misunderstandings and expectations (1 Cor. 7:3–5). Life hasn’t changed much since. It’s a surprising reality young couples need to be prepared for.

5. The Parents/In-Laws Surprise

Marriage shuffles your relational network. No one feels it more than your parents. Jesus said, “For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh” (Matt. 19:5). God designed marriage to create new families. And to start one family, you must leave another.
Typically, people reduce this to geography: “I’m moving out of my parent’s house and in with my new wife across town.” But “leaving and cleaving” also alters your parents’ authority and responsibility. Once a couple gets married, there’s a seismic shift in the parents’ role. They don’t stop being Mom and Dad, but they can’t expect to be honored the same way they were when the kids were young. How time is spent, the frequency of being together, where holidays happen, expectations for seeing grandchildren, the way counsel or opinions are shared—all of these glorious blessings must move out of the realm of expectation and into the realm collaboration.

6. The ‘Forgiveness Is Costly’ Surprise

“Everyone says forgiveness is a lovely idea,” C. S. Lewis observed, “until they have something to forgive.” Perhaps the most painful and courageous part of forgiveness is when we must absorb the cost of a spouse’s sin. The pain of being sinned against doesn’t go away quickly. Words spoken, money lost, vows broken—these pains get stuck on “repeat.”
Heartache and mental anguish can break into your mind unannounced. It creeps up when you’re down and can greet you the moment you wake. But biblical forgiveness absorbs at least two costs. First, a spouse must say, “I’m not going to punish you.” There’s not a person among us who hasn’t mentally prosecuted a spouse and delivered the verdict spoken by the unmerciful servant in Matthew 18:28: “Pay what you owe!” But for forgiveness to happen, we must deny our instinct to throttle a debtor and release them from punishment.
Second, we must say, “I will pay the debt for this sin instead.” See, debt doesn’t just mysteriously evaporate. If I loan you 10 bucks and you refuse to pay, the money doesn’t magically appear back in my wallet. Someone has to eat it. This often trips up reconciliation. We want to forgive, but we assume it shouldn’t cost us. We feel that sheer willingness to not retaliate is sufficient. We instinctively react to the injustice of absorbing a debt: “You did it! Now I pick up the tab?” To treat our spouse as their sin deserves (with anger, withdrawal, or emotional punishment) seems more fair and equitable. But when you do this, you’ve forgotten just how much you’ve already been forgiven. You’ve forgotten the debt Christ paid for you. You were forgiven a great debt. Marriage often means doing the same.

Remove the Blinders

Many young couples head into marriage with blinders—believing their marriage will be the fairy tale they dreamed of as they planned a Pinterest ceremony and momentous honeymoon. But the truth is marriage reveals our sin, exposes our desires, challenges our relational network, and requires us to regularly practice costly forgiveness. Engaged folk need to know that marriage is a call to ministry where two sinners learn—till death parts them—how to apply the gospel of grace.
If you’re a pastor or premarital counselor, tell them about the surprises that marriage will inevitably spring. It will prepare them for the greater wonder of how Jesus works through broken people to reveal his matchless love (Eph. 5:31–32).