Today is my wedding anniversary. Catherine and I have been married for 30 wonderful years. It is a joy to be able to say that our relationship has grown better every year. It is not, however, because we are loving each other more each year (though that is true). It is rather because we are both working to love God better every year. This is certainly not something that is easy, but it is something that a Catholic couple can help each other to do. In short, I rejoice this year because we have helped each other to serve God better, and that enables us to serve each other better.
There is a very good reason why the Catholic Church says that it is a "no-no" for Catholics to marry non-Catholics. Yes, I am sure you have probably heard about it happening all the time (and possibly have done this yourself, or at least might be related to someone who has). I have had people say to me directly that they think it is a bit "silly" or even "unnecessary" for the Church to say this. Yes, it is true that a dispensation can be granted for a Catholic to marry a baptized protestant, but it comes with significant warnings about the challenges to such marriages. It is also the case that a dispensation can be granted by the Bishop for a Catholic to marry someone who is not baptized at all, but that is strongly discouraged.
You might be asking "why" this is the position of the Church. It is not just the practical consequences of these unions, though that is significant (cf. 2 Cor 6:14, where we are told not to be "mismated with unbelievers"). The statistics show that the vast majority of Catholics who marry an unbaptized person with either end in divorce or the Catholic will abandon their faith. Yet, there is much more than that involved. It has to do with the impact that such relationships have on those around them (the children, as well as the larger community around).
Here are some basic points to consider in this regard. Firstly, an unbaptized person (pointedly referred to as a "pagan") does not have a Catholic concept of God or the love that He requires of us for holiness (if he did, he would probably cease to be a pagan!). Also, a protestant may have a generally similar view of God and love as the Catholic, but there are going to be some differences, often in crucial areas, that will impact the relationship. If the Catholic spouse and the non-Catholic are not working with the exact same perspective on God and His laws, then there are going to be points which we would have see as "irreconcilable" since they are viewing the world from two different perspectives.
Secondly, when a Catholic marries another Catholic, then we can presume that they both have a Catholic world view, and that if there is discovered a point at which that is not the case, then we are able to call the errant Catholic to account for not following through with his or her Catholic faith. This cannot be done with a non-Catholic. I cannot say to a Buddhist, or a Muslim, or an atheist that he is responsible to believe and behave like a Catholic. The very idea is foolish. There is no agreed-upon standard to draw him back to.
Thirdly, when two Catholic spouses are properly focusing on God and His words in the same way, then both can find personal fulfillment in the proper manner. Only Catholic teaching fully upholds the biblical principle that human happiness and fulfillment can only be found in God. This means that the Catholic husband and wife are both seeking their primary fulfillment in God, and their consequent joy in each other. If anyone (Catholic or not) seeks their primary fulfillment in their spouse, then they are going to be disappointed (and thus either seek their fulfillment elsewhere, which always leads to either infidelity or divorce). If a married couple is looking to each other to find fulfillment only frustration will be achieved. When they both realize the One and Only True Source of personal fulfillment, then, and only then, can their relationship grow properly into holiness and joy (which means more than just "sticking it out" in their marriage).
Some might think that this is just a utilitarian understanding of marriage; like saying, "seek a selfish goal in the right way and you'll be OK". It is selfish to seek one's personal joy in the wrong ways, but since the right way is to seek the righteousness of God first, then it is not selfish if the consequences are one's own happiness and joy. Yes, there are some marriages that last for decades without this proper perspective, but I would question whether they are genuinely enduring and growing closer to Christ, or whether they have merely found a way to let each other grow their own way while staying under the same roof (which is all too often the testimony given by many in this situation).
We must aim at much more than dysfunction or mere tolerance of one another. We must be aiming at holiness. We must be aiming at the spread of the gospel to the future generations that will follow in our footsteps. With marriages so weak today, let us make sure that we follow the right path to achieve our goal. Jesus offers us His strength, and if we receive it, then it will lead to our eternal joy.