It usually depends on how the couple feels.
I have a few friends who are currently living or used to live with married/unmarried couples.
One of them was living with a married couple for a few months. When that married couple had kids, he decided to give them space and moved out.
Another friend is currently living with a married couple right now, but it's for money reasons since they are still living in this town. That couple plans to move elsewhere in a few months, so my friend is taking the opportunity to find a job and move elsewhere on his own or with another roommate.
In both situations, it was worked out with the couples.