Let me guess, all of these men who are so principled as to refuse to buy diamonds still conveniently want their fiancé to look good. To dress nicely, not be overweight, shiny hair, nice skin. This requires some money, time, and effort.

Heels will damage her feet over the long run, clothes are over priced and expensive, the makeup industry causes all sorts of evil just like diamonds. But you'll want her to look good the way your culture defines it, because it signals your value socially.

But diamonds are horrible and women are selfish and irrational for wanting them.