Infidelity is also human nature. Is that also just to be expected in a relationship?

If someone isn't willing to compromise in the face of reason (e.g. "we can't afford an expensive wedding or a ring", or "I'm concerned about supporting conflict in Africa and supporting the diamond cartel") then I'd say the relationship is probably not strong enough to last.

But it's not as clear cut that this is a 'negative' aspect of human nature, and I would strongly argue that infidelity is NOT necessarily human nature, at least not as fundamentally as gift giving / signaling.

And you're moving the argument here - previously we were talking about buying an expensive ring, not supporting armed conflict or purchasing something you can't afford.

Given the state of the economy and the average savings rate in the US the "typical" diamond engagement ring is simply not affordable for many people. More so, the idea that acquiring a diamond through the De Beers cartel does not at worst indirectly support conflict in sub-saharan Africa and at best help prop up brutal dictatorships is just extraordinarily naive.

Human nature only talks about the average, I know a few people that dislike expencive gifts as it makes them feel obligations that they dislike. Gift giving is a vary cultural activity which makes it seem a lot more innate than it is because most people you interact with share the same bias.

Absolutely. I'm not saying it applies to everyone, in fact I'm completely on your side - disagreeing with (and demonizing) giving a diamond because it's not how you operate is silly.