My girlfriend was telling me that cohabitation was illegal in 5 states. I’ve never heard of the term “cohabitation” before, it seems like it must have a vague meaning. What exactly is cohabitation?
“Seven states still have anti-cohabitation laws on the books, but they are almost never enforced and are now believed to be unconstitutional since the legal decision Lawrence v. Texas in 2003.”
Two people of opposite sex living as a married couple without legally being married.
It was formerly illegal in many jurisdictions, and may still be on the books in some. After a certain duration it was often recognized as “common-law” marriage.
Ah, you young people are so cute when you say stuff like that. 
Was?
If you are living as “boyfriend-girlfriend”, does that count?
In most states. Only 11 states and the District of Columbia still allow common-law marriages. Twenty-six states no longer allow it, and 13 never had it. (Although those states recognize common-law marriages from other states.)
In modern terms, yes. If you are having sex, you are “living as man and wife” (in the traditional sense).
From what I’ve heard from most married couples is that they don’t have sex. If I’ve heard correctly, sex stops as soon as you get married, typically.
Yup.
POSLQ. Persons of Opposite Sex Sharing Living Quarters.
Ha ha. You haven’t heard correctly, or else your friends are atypical. The average frequency of sex among married couples in the US is a couple of times a week.
Shacking up.
I suspect the laws were easily circumvented by claiming the lady was your sister. A brother 7 sister sharing a home wouldn’t raise alarm.
I knew a guy that got the cops called by a motel on his wedding night. They were both 17 (legal in my state) and checked into a motel after the wedding. The motel clerk thought they were local high school kids shacking up and called the cops. They had to call home and prove they married. this was 1973.
For a common law marriage (in those jurisdictions which still recognize it), in addition to cohabitation, the parties must intend to consider themselves married and hold out this marital status to the public. (The public, in general, deem them married. If you ask the next door neighbors if they are married, and they say that they are just living together, then there is no C/L marriage.) They must sign all legal documents (deeds, insurance papers, etc.) as husband and wife.
IANAL.
IIRC, There is a similar concept of a common law name change - if you consistently use a name other than that on your birth certificate or marriage license and hold it out as your name, and you become generally known in the community by that name, it becomes your legal name for all intents and purposes.
You can adopt any name you choose so long as it is not used for fraudulent purposes.
Down to at most five. Virginia Supreme Court struck down their co-habitation law in 2005, and West Virginia law was repealed in 2010.