Complex sentence: Common-law marriage is an option pursued by many as a form of resistance to traditional institutionalized marriage; however, in this context, states reserve the right to define the relationship as marital, based only on a history of co-habitation.
Split into simpler sentences: Common-law marriage is an option pursued as a form of resistance to traditional institutionalized marriage. However, in this context, nations reserve the right to define the relationship as marital, based only on a history of co-habitation.