How is that equivalent? If I'm a vegan, I don't need to pressure others into my own belief system and expecting them not to eat animal food as well (I could, but I might as well not). Expecting your future spouse to buy a diamond that they think is an irrational purchase is by definition forcing your beliefs on others.

To be honest, so is not being willing to buy a diamond for your partner that wants it, but that's the point, find another person who agrees with your expectations of them.