Having kids will change your life. I've mentioned elsewhere how this happens, but don't take my word for it. Just observe any parent at church and you'll see.
In conversation, you'll notice parents not making eye contact with you because they're also watching for their kids. During lunch, you'll notice parents feeding their kids first before taking a bite out of their own plates. And going home, you'll notice parents first loading their car for about ten minutes before they can leave the parking lot.
Having kids will change your life. Your routine, schedule, living situation...it's all different. In fact, you and your spouse will be different. Kids don't just change your life, but they also change you. And if kids change you, it makes sense to conclude that having kids will change your marriage. But is it for the better or for the worse? Do they often help or hurt your marriage?
From my experience most newly weds are afraid to have children, which is often why they wait until 2-4 years before having kids. Sure they plan to have kids one day because, well, they feel like they should. But until then, they want to "enjoy marriage," which means traveling and going out - just the two of them. And they know having kids will likely change all of that for them.
In other words, a lot of newly weds see kids as a potential problem to their marriage. Of course they'll never say this because children are supposed to be a blessing. But they also know that kids can really hurt the quality of a marriage. After all, they see their friends with kids and they shudder. Though their friends seem happy as parents, they don't seem happy as spouses.
On the other hand, there are some married couples who see it another way. They view kids not as a potential problem to their marriage but as a potential solution. You see for some new couples, marriage is tough; they fight or feel disconnected from one another; they feel bored and apathetic; they have issues that aren't going away. Marriage is simply not what they thought it would be.
Therefore some couples see kids as a way of helping them with this problem. After all, not every parent looks miserable. They see the connection that develops between two people who journey together through conception, pregnancy, and childbirth. Such an experience may cause marital problems to look a bit trivial. And if most people do change after having kids, can't this possibly reignite the passion in their marriage?
Helpful or Hurtful? Well, It's Kinda Both
So how will children affect your marriage? How do we make sense of some marriages looking terrible after kids but some marriages looking great with kids? Well, here's a conclusion that I've arrived upon: Having kids will often make a good marriage better and a bad marriage worse.
If your marriage is great and things are going well between you and your spouse - fear not. Children will not "ruin" things. In fact, they will only enhance the joy that you're experiencing with one another. Of course kids will make you more tired, but they will unite and bond and grow the two of you closer than ever before. You've been enjoying steak (marriage) and now you're about to add some amazing seasoning to it (children).
But if your marriage is struggling and falling apart - be patient. Children will not save your marriage but will only amplify/suppress the problems. Kids will prevent you from working on the issues you need to work on because you're busy caring for the kid. So while you're working together, you're potentially more divided than ever because you're talking only about your kids and not about the issues in your relationship.
Why This Is Often So
In the Four Loves, C.S. Lewis talks about how healthy friendships often feel richer when you add another person because, in a sense, you're inviting that person to share the joy that the two of you are already experiencing. The third party isn't invited to displace the friend; rather they come to participate in the fun that's taking place.
If marriage has a healthy "friendship" element to it, kids will not hinder the relationship; instead they will enhance it. Before having our son, my wife and I would often go to Downtown Disney to enjoy the restaurants and stores. But now that we have a son, we're even more excited to go downtown because we also get to see him enjoy the restaurants and stores. Of course we may not get to see as many stores as we'd like or eat as peacefully as we used to. But such limitations pale to the joy we get in seeing enjoy what we're experiencing.
But I could imagine if we had a son during our difficult season of marriage (see this), we'd probably use him more as an escape. After all, we're not inviting him to join us in the joy that we find in one another. No, we're looking at him to fix us - and that's not only unfair but unrealistic because healthy relationships just don't function that way.
For married couples who are experiencing difficult times, I'd highly encourage them to work hard at their issues before having children. Once kids come into play you won't have nearly as much time to resolve those issues. I'm not saying things need to be perfect before having kids, but the more issues you resolve the less your kids have to experience the consequences of them.
For married couples who are experiencing blissful times, I'd highly encourage them to have kids sooner rather than later. I'm not saying newly weds should shack up right away. I do see the wisdom in taking a few years to really get to know each other in this newfound relationship. But if things are good, don't fear kids will ruin things. They won't. They'll only make things better.
It's ironic how this works. Couples who think kids will fix their broken marriage will usually find that kids just break it even more. Conversely, couples who think kids will break their healthy marriage will usually find that kids will just enhance it even more. Again, kids will be a personal blessing no matter what. But how they affect the quality of a marriage? That usually depends on the quality of the marriage beforehand.