It seems to me that you would treat it as your family typically would. Families are all different and I'm sure you would use common sense. It basically boils down to closeness of the relation (grandparent vs 2nd cousin), genders, and ages. Regardless of being Traditional Catholic, you trust in God and try to do what is right. Go back a couple hundred years, in different countries, and you will find many large family living in 4 room (or smaller houses). My 3 bedroom house was built in 1910 and was home to a family with 9 children. I'm sure they had people over for Christmas. Just do what's right and don't overthink it.
This was your response even though you thought he meant "family members living together"? Huh?
There is absolutely nothing wrong with family members living under the same roof! I'm currently living with my wife, 6 daughters, and 1 son. Not even a
fault or
imperfection, much less a sin. Moral theologians would say: NO SIN.
Cohabitation means a man and a woman (unrelated) living together who are not married. Fornication is NOT necessarily presumed. Cohabitation (as defined in the previous sentence) is mortally sinful because it is an unnecessary, PROXIMATE (near) occasion of sin for the parties involved. It is also scandalous.
I don't think it applies to "house guests", unless the wife or husband is alone in the house, and the "guest" is a member of the opposite sex.
Cohabiting can be summed up by the old-fashioned slang term, "playing house". If someone wouldn't joke that a couple is "playing house" (pretending that they are married, living in common together), then it isn't cohabitation.
2 men or 2 women living as roommates is also not cohabitation. Perversion, or being seriously twisted/mentally ill (the real definition of ɧoɱosɛҳųαƖ), is not presumed, nor SHOULD IT be presumed.