A relationship base on just a verbal agreement is that, just words. Words can easily be blown away by the wind. You can break up at any moment for whatever reason. You can leave even the day after you've just professed your love for them. And what were all those vain words for? For nothing. If they cheat, you can easily break-up and the cheater can even lie saying it never happened, that you're lying and you got no proof other than just your words against theirs.
Do words alone bring you security and financial stability? If your partner were to suffer an accident, they can't speak at that moment and someone had to make medical decisions for him/her, would ''but we love each other'' and words be enough for doctors to give any rights for their health? Would ''but we love each other'' be enough for you to be included on their health insurance through their employer?