Census PUMS data includes a variable PARTNER identifying unmarried partner households, either same sex or opposite sex, but I do not find any corresponding variable in IPUMS-USA.
We do not currently offer a variable corresponding to PARTNER. However, IPUMS provides a code in the person-level variable RELATED (the detailed version of RELATE) called Unmarried Partner, and the code is 1114. For more background information about how this variable is constructed and approached, this paper has a detailed description: http://economics-files.pomona.edu/steinberger/research/Gates_Steinberger_ACS_Miscode_May2010.pdf