The marital status question has 6 possible answers: Married, spouse present/Married, spouse absent/Divorced/Separated/Widowed/Never Married. How does cohabitation considered in marital status? For example, if you consider a family where the parents live together with their child but are not legally married - how would the wife list her marital status?
The IPUMS-CPS variable MARST represent a direct response to a question from the CPS questionnaire, though the variable is edited in some samples by the Census Bureau (see the “Data Quality Flag” QMARST). Presumably, if a couple lives together and has a child together but are not married, the couple would not be identified as “married”. Starting in March of 1995 the RELATE variable does identify “Unmarried Partner” of Householder and starting in 2007 the variablePECOHAB identifies the line number of the respondent’s cohabiting partner (PECOHAB has not been edited for accuracy so it is recommended that users take steps to check the validity of the values).
I hope this helps.