Nephites had been marrying and being given in marriage since the beginning of their society. There was nothing unusual in marriage. In fact, Mormon puts the statement here precisely because it is normal—a continuation of normal life during these pseudo-millennialistic years.
Reference: The language of verse 11 borrows its form and vocabulary from the New Testament:
And Jesus answering said unto them, The children of this world marry, and are given in marriage:
But they which shall be accounted worthy to obtain that world, and the resurrection from the dead, neither marry, nor are given in marriage. (Luke 20:34–35; see also Matt. 22:30, Mark 12:25)
The contexts are different, despite the similarity in vocabulary. There is parallel language in Luke 17:27 where the concepts of marrying and being given in marriage also indicate normalcy.