Trying to argue against engagement rings is like arguing against internet censorship. Your opinion is tainted before you open your mouth because the only people who could possibly oppose such things are cheap bastards or pedophiles, respectively.

If you refuse to buy a diamond, it's because you're cheap or don't love "her" enough. No amount of hand waving or sensible logic will make up for the fact that you'll just look like an asshole.

There's no logic in love, just shut up and buy her the damned ring.