How old was the missionary? Was the missionary 18? Was the missionary gay?
(You talk as if this was carried out in the context of marriage...it wasn't)
Even within marriage...
Per http://www.wisegeek.com/how-has-the-average-age-at-marriage-changed-over-time.htm: If one looks at US statistics over the past 100 years for example, one sees that men had an average age at marriage of 25.9 years in 1900. Women in 1900 had an average age at marriage of 22 years. For some this shatters an illusion that women 100 years ago were sold into marriage as young children. Even Jane Austen, writing in the early 19th century had heroines married at the earliest age of 17 or 18. In Laura Ingalls Wilder’s books, which are semi-autobiographical, her father would not allow her to marry until she was 18. Thus it can be said that the average woman was past 21 when entering her first marriage, 100 years ago.
Not 18...crime occurred in 2012...
Lds Church didn't allow 18 yo missionaries in 2012.