text: No, I don't mean 'slippery slope', as I'm not sure what the next step would be; it is already a broken promise, and loss of fidelity.  You're describing communal living, as in a group of people living together as family for the mutual benefit of all, not necessarily sexually bonded; potential new members are vetted by all present members, to fit into the whole.  Communal living can provide a stable, loving environment for children, and needed security.