some states do have common law marriages..
after x period of time,
a couple is seen as such
by courts
if they meet certain requirements
(each state has its own laws)
most typically a couple has to have
presented themselves publically
as a married unit:
such as shared bank accounts,
living together,
perhaps even taking on shared names,
promoting themselves to others as husband/wife

there are court requirements that must be met,
should they separate and one attempt to gain a portion of
the others assets because of shared living,
they first have to give proof of many things,
to show that they were not just *roommates* etc.

IF Terri Shivos' husband can be shown to meet that States
guidelines for commonlaw marriage,
then there would be a potential situation
that her family or the state
could prosecute him for polygamy yes.
unless they are mormon and live in Utah.