Yes one could cross state lines to marry from where a bond was posted. They were just a way for a family to guarantee a legal marriage would take place. When a legal marriage certificate was presented to the county that held the bond the money was returned. If you look at the Campbell Co entry for Stewart and compare it to others you will notice it only has a BD (bond)issued and bm was Elizabeth Gordon.Other persons listed also haveMD (marriage) with a later date which means the marriage took place within that same year. So no marriage took place in that year that was recognized by Campbell county as legal for Stewart/Gordon. Either they did not marry or had to marry again in 1881 to get bond money back. Have you found them in the 1880 census? Are they married ?