You might also want to go further to add...how will she explain to her friends and family why she choose you over some other better guys that is there for the picking. Would you want her to defend you...her choice ...over what others have to say about it?
If a woman give in to that kind of notion and would cave-in and be made to feel bad that she is not wearing a diamond ring after she is married...well...what else would she succumb to at the mercy of other's comments and hearsay. It will not stop at just the ring comparison don't you think? A relationship is about two person no? What you can afford to give or not give...if you two love each other that much to get married and make a life togeher....why should the size of the rock be that important.
Another thing....I don't often see lots of married ladies at work and in social event spotting diamond rings. Wedding gold bands yes but rarely a diamond one.
Anyway...no right or wrong...just choices we make and will have to live with.