The "issue" is child rape.
What were the legal demands of statutory rape when your parents were married? There are a LOT of 21-year-old guys facing rape charges and in prison for sex with 15-year old children.
In the mormon area where I was raised, a nineteen-year old mormon man impregnated a thirteen-year old and they married, and there were several cases of girls under 16 marrying their older boyfriends, but that was decades ago.
If one can't listen to the Jeffs' tape and know that this was a case of rape, they have a skewed vision of the term.
The arguement here is an attempt to make the exception the norm. It is NOT normal for young teens to be married in our society, either now or in the past - such were the exceptions. It is similar to saying that just because one person survived going over Niagra Falls in a barrel, everyone can and it is normal - the fact is that it isn't.