There are two kinds of church growth: births to Christian homes and converts.Globally, births in Christian homes are by far the larger number (about 10x: 45 million births vs 15 million converts and losses of 12 million defectors per year).
When a church faces a situation in which it there are fewer and fewer births to Christian homes, it loses a significant form of church growth, and its share of the population will begin to shrink.
This is one problem in smaller churches (such as Turkey, Iran, Egypt), where believers find it hard to find other believers to marry, and often leave the country.
But it can also be the case in the West, as this article in Christianity Today points out: How the dating scene became stacked against women (and especially college-educated religious women).
Megachurches in this respect may succeed more than smaller churches, because they have larger male/female pools and more effective ministries to singles (both in helping them sustain their faith and, for some, helping them find matches).
Christian and quasi-Christian matchmaking websites (e.g.
eHarmony and their like) can also help church growth in this respect. It’s important for its future for the church to help believers to marry believers and raise believing children.