If there are so many American Catholics, why aren't they getting married?

In fact, this is a major reason why people have historically converted to the Church as adults. According to a 2009 Pew study on religious conversion, 72 percent of adult Catholic converts said that marriage was an important factor in their decision to change their faith. For those who convert to other Christian denominations, the reasons tend to be more connected to doctrine and belief: People who convert to Christian denominations other than Catholicism are more likely to say, for example, that they "drifted away from [their former] religion" or they "stopped believing [their former religion's] teachings." According to Mark Gray, the director of polling at the Center for Applied Research in the Apostolate, however, Catholic converts tend to cite expectations placed on Catholics when they get married.

"The rule is, if you're marrying someone who isn't Catholic, the Catholic spouse has to be open to having children and commit to raising their children Catholic," Gray said. "The non-Catholic spouse very often is Protestant, and they often choose to be a member of the same faith as everyone else in their family."

But even though marriage has been a major reason why adults have joined the Church in the past, it’s becoming less so. Between 2000 and 2012, adult baptisms declined by nearly 50 percent, which, Gray said, probably has something to do with the declining rates of marriage.

So why are couples choosing to get married outside of the Church? For one thing, there might be a lack of awareness about the specific doctrinal importance the Church places on marriage. "More people are choosing to get married in country clubs and at the beach," said Gray. "A lot of people are unaware of the importance of marriage and the place it has in Church sacramental life …Younger Catholics are probably not going to have a deep awareness about the sacrament of marriage, even if they self-identify as Catholic and [have] religious beliefs."

Meanwhile, fewer Americans are getting married generally. From 2000 to 2011, the number of marriages in the United States fell from 2.3 million to 2.1 million, or roughly 0.82 percent of the population getting married each year to roughly 0.68 percent.

But while that's a factor in declining Catholic marriage rates, the deeper cause is the changing relationship between people and traditional institutions, Gray says. "It's not just churches, but all kinds of institutions have experienced detachments from the 'brick and mortar,'" he said. By “brick and mortar,” Gray means in-person religious communities: people physically coming together in churches to worship together. In general, Americans are becoming less affiliated with traditional religious institutions, not just the Catholic Church: From 2007 to 2012, the percentage of the religiously unaffiliated grew from 15 percent to 20 percent, according to the Pew Research Center.