As long as they're not militant, they wouldn't care about marrying a person of a different religion. If they love each other, then so be it. They shouldn't try to convince the other that they're right though, but they may have to raise their children in a secular household. Lol.
If they are in love, and both of them agree with each other, they should be able to marry. If the atheist wants to marry a catholic, so be it. The catholic should not have to make the atheist go to church to be brainwashed into believing God, but the atheist should not try to convince the catholic to be atheist if that is their religion and they would like to stay that way.
I have to say that marriage encompasses a lot of factors and is not isolated only to religion. I believe that an atheist can marry a Catholic because of the psychological and physiological need of having a partner, which can be attained without the inclusion of religion in the issue. Procreation is an objective of marriage for both religious and social responsibilities. Since. we can not include religion in the picture, they have to fulfill the responsibility of procreating in the grounds that is a social responsibility and personal contentment. Many people around the world practices inter-religious marriages and became successful. Religion or belief is sometimes the reason why people misunderstand each other that is why if an atheist marries a catholic, they have to set aside what they believe in and have to be connected on human level, excluding matters of belief or religion.
There are many instances of differing beliefs in marriages that have been talked and written about before now. It is possible that it could lead to difficult situations in regards to the upbringing of the children and religious practices, but it is the love that is important. If they truly are meant to be together, they will be able to talk and work through their differences in a mature way so that there is no misunderstanding regarding their differing beliefs.
Children brought up in a household where there are multiple viewpoints get to experience the thinking and example of a broader view point. The danger comes when one parent decides to indoctrinate the children with one view or another. It is very easy for the atheist, of this question, to pay lip service to superstition, when ceremonial occasions arise. The most objectionable thing about marrying a Catholic, for an atheist would probably be that they are required to agree to raise their children as Catholics, or that they may need papal permission. But to an atheist I suspect that promises they make to the church are not actually binding on them.
Although there are two opposing viewpoints, there's no reason an atheist and a Catholic can't get married. People disagree about a great many things, and even though for me personally matters of religion and superstition are dealbreakers, there are plenty of couples who make it work with surprisingly big gaps in thought processes, like married couples of different political parties.
The Bible is clear about this. 2 Corinthians 6:14 says: "Do not be yoked with those who are different, with unbelievers.[b] For what partnership do righteousness and lawlessness have? Or what fellowship does light have with darkness?" (NAB). If your goal in life is to become more holy and raise children who are holy, it makes absolutely no sense for the primary person in your life (and one of the few you get to choose) to be either diametrically opposed to that outcome or pursuing some other path.