One reason not stated by the author is that U.S. culture and tends to handle badly the ebb and flow of physical intimacy among co-residents and their guests, not only through the excuse of jealousy as a conduit for physical and emotional abuse, but also by applying the playground puritanical ‘cooties’ logic to (almost exclusively) women who have been ‘contaminated’ for future relations by a prior partner within the community. Not that communes uniformly handle this any better — jealousy and power plays can still tear up an open-partners community! — but as all constant human coexistence groupings such as work, school, and churches demonstrate: where people are around each other often, intimacy and its successor attractions will crop up without regard for monogamous fidelity. So, given the U.S. puritanical tendencies, it makes sense that they avoid coresidency: isolating humans inhibits (somewhat) intimacy outside of the married partner.