5. All responsibilities don't fall onto one partner.
Despite the best of intentions, once a couple cohabits or marries, one partner usually ends up shouldering more of the attendant responsibilities. In almost every case, there's going to be one person doing more of the things. Those things aren't just household chores, either.
Research shows that marriage often shifts the responsibility for all sorts of things, like managing social commitments and maintaining extended familial relationships, disproportionately to one partner. The fix? Don't get married.