1. a person with whom one shares a house or other residence.
2. a sexual partner with whom one shares a house or other living quarters without being married.
However, I have never heard or seen the word used in that second sense. I would normally expect the a term like domestic partner or cohabitation to express that meaning, not housemate. Furthermore, I could not find another dictionary with a similar sense of the word. Only Urban Dictionary has a related entry, which appears to be a joke and not a serious report of usage (“My friend just housemated with my fellow housemate”).
Is the Dictionary.com entry spurious, or is the “sexual partner” sense of housemate in actual use? If so, is it regional or perhaps euphemistic?