Are they on the 1871 census as a married couple? If so - marriage was before census date
Not necessarily; for example (in my own family):
1911 CensusTom Francis Greenland and Elizabeth Greenland
- married in 1937, after respective first spouses had died.
7 April 1861 CensusEdward Charles Green and Sarah Mary Green (false details)
- married under their real names (Charles Greene and Selina Greene) on 13 May 1861, after Charles's wife "drowned".
31 March 1901 CensusWilliam Allum and Elizabeth Allum
- married on 29 July 1901, this being Elizabeth's fourth bigamous marriage.
1901 CensusEdward Ware and Florence Ware
- never married at all, Edward probably believing he was already legally married to Elizabeth (shown above as Allum).