I couldn't agree more. It might sound idealistic but I couldn't imagine being happy in a relationship where we don't agree on core principles (such as a stance on buying expensive diamonds).
I couldn't imagine being happy in a relationship without learning to compromise. Only the most important things should be considered "core principles", questions of religion, child-rearing, maybe hot-button political issues, and other moral litmus tests.
Whether or not she gets the ring she wants is a silly gender identity fixation, pretty harmless in comparison. If its a core principal, then so is whether or not he's allowed play video games, or buy a motorcycle, and so on. (Btw, I'm not engaged, married, nor ever bought a ring.)
I agree, compromise is incredibly important and it's all about finding someone whose core principles are the same as yours. For me, the thought of having a partner who would argue and resent me over me not buying an expensive diamond ring would suggest a clash in core principles. But I completely appreciate that that's my own view of it.
I may change my mind to make her happy. She may change her mind because our financial situation has changed (mortgage). We may decide not to get married at all.
I don't consider this a core principle. Our views on the important stuff are very much aligned for the most part.