When I was in the Army, late 60s, shacking up was considered conduct unbecoming.
You could get an Article 32 for it.
I graduated Medical School in 1985 and shacking up was not accepted then either. I still do not accept it. Now people live together and move from one relationship shacking up to another. I do not understand why any woman would agree to those terms. I never would. I was in my second year of residency and my future husband was going to have to go to NJ for 2 years of residency. I told him if he wanted me to go we were going to get married first. We did and we are.