Yeah, I'm curious about that attitude. Not personally examining the grandparent (to whom I wish a long and happy marriage), but is that a real thing, that a woman would break off a promising life because of the lack of a diamond ring? If she would, does that indicate that she doesn't know how to otherwise value a mate, or sees it as an overriding signal of this mate's suitability (even over rational things like job prospects, attractiveness, health, kindness)?
This has come up before and people on HN seem adamant that women will dump you if you don't buy them diamonds. My suspicion is that either this theory hasn't actually been thoroughly tested, or it's to do with the kind of women the men of Silicon Valley tend to seek out.
