Taken to its logical conclusion, your argument boils down to saying that any human who spends money on something that's not strictly utilitarian deserves to be treated as non-human. You try to hedge against this by saying that you enjoy
some traditions--like putting up a Christmas tree--but that just shows bias. A Christmas tree is a pointless item that people buy because it looks nice and the ritual helps bring people together. Why can't a diamond ring be bought because it looks nice and can help bring people together? There are definitely people out there that actually enjoy shopping for the ring.
There are many things people "waste" money on, yet they don't deserve to be called sub-human or property. People pay thousands of dollars extra for luxury car models. People pay millions of dollars for art that they can't appreciate any more than a dollar store painting by Joe-painter. People pay hundreds of dollars for expensive meals that contribute to the suffering of animals. These are the extreme cases, but every human being that has expendable income will buy pointless things simply because they like them. Heck, weddings themselves are usually more expensive than the ring, and they're an even greater waste of money; at least you can sell the ring for something later on. People pay for memories and for experiences (which the ring also provides, by the way); compared to those things, which hold zero financial value long-term, overpriced gemstones are actually a relatively good investment. It's more like buying an overly huge TV than setting the money on fire.
Yet once again, even if they were the worst investment in the world, your pride in taking a dehumanizing stance is bothersome.
I don't know why you would think the woman is contributing nothing to the relationship. Why not assume the man is contributing nothing? Maybe the woman is the breadwinner and highly supportive while the man pursues some artistic dream that generates little income? Forget the finances; why can't a woman contribute mentally and emotionally to the relationship? I simply can't understand where your comments are coming from.
The most troubling part of the tradition is, of course, that the man is expected to do it. Except the woman is also expected to receive it. If you personally refuse to buy a ring, then you need to find a partner who agrees with your stance. But, even a woman who doesn't care that much might still be put into uncomfortable situations (by friends, family, etc.) by not having one. To her, the social inclusion might mean she would prefer to have one (plus, it looks nice). If your partner makes it clear to you that they value a ring, then there's no foul play; it's simply one more part of their complex personality. It's simply an emotional preference; intelligence doesn't come into it. You can choose whether that's a deal breaker for you. Personally, I'm happy to buy the ring, because I enjoy buying nice-looking things for other people, and the ring is one I'll get to appreciate every day I see it. Like having children or living in Hawaii, it's something that one person might value more than the other, and it's important that the couple come to an understanding before they commit to each other.
Your fixation on the 3-months salary is misguided. First off, that's a guideline, not a requirement (I've heard of few brides who count the pennies, and the ones that do make their values quite obvious). Second, I seriously can afford to spend 3 months of my salary on a luxury item. I understand and sympathize that not everyone is as comfortable a financial situation as I am, but just like every single purchase, how much of a "burden" it is to you depends entirely on your financial situation.