Girls don't "need" anything except food, water and shelter, but a man who gives her only that won't last very long. A blanket statement like "I would never buy you a diamond" implies "I will vet all your purchases." It's the most un-romantic thing one could possibly say. It sets the tone of the relationship, something like "What you want has to make sense to me before I give it to you."

It sounds quaint and old-fashioned, but a man should give everything to a woman he hopes to marry, and demand nothing in return, without worrying about whether or not it's sensible. It doesn't make sense until you realize that she only wants to do the same for you. She will commit to unconditional satisfaction of your wants, but you have to commit to hers as well, and saying "I would never buy you a diamond" sets up some major conditions.

It's not about the diamond. You can debate the diamond specifically (and btw, the showing-it-off part is where most of the worth lies) and you might win. It's about the debate, about having to discuss her wants in order to decide if they make sense. That is unromance.