She deserves to be treated as a possession because somehow the man is expected to shell out all of HIS money for this stupid rock, solely for her benefit. What exactly is SHE contributing to the relationship anyway? Obviously, she has nothing to contribute except being a possession, so she should act like this, because this is the role she is demanding. After all, why isn't SHE blowing 3 months' worth of her pay on something for you?
Is this dehumanizing? Damn right it is. If you demand a dehumanized relationship, then of course it's dehumanizing. If you don't want to be dehumanized, then demand a relationship based on equality. I'm not going to treat a woman as an equal if she doesn't act like it, and instead demands to be treated as some kind of possession, which is the only reason there is to spend 3 month's salary on a stupid rock for her.
And yes, it absolutely IS a marker of unintelligence for her to not reject this idiotic tradition. What do you think of women who demand to have their or their daughters' genitals circumcized because of tradition? That's pretty stupid too. This is no different. Again, if you want to be treated as an intelligent person, then start acting like it. There is no rational basis for wanting a diamond ring worth 3 months' salary. And while I may not be perfect as far as rational preferences, I try damn hard to make sure I am rational and am not clinging onto stupid beliefs for no good reason. This also isn't to say that all traditions are stupid: for instance, I like the tradition of putting up a Christmas tree (which of course comes from Germanic pagan traditions). It looks nice, it's seasonal, it's fun putting up the ornaments with family, etc. And these days you can get artificial trees so you're not killing trees for the tradition (you can also get trees with intact root balls and then replant them after the holidays). But the important key here is cost versus benefit: holiday traditions are fun and a nice family practice, but they're also pretty cheap. Artificial trees and ornaments are not at all expensive these days, unless you're desperately poor.
Now, as for your comment about a ring being a burden, it damn well is a huge burden, for everyone. Did you forget the part about it having to be 3 months' salary? This isn't like a Christmas tree which is generally fixed-price, it scales to whatever your income is. Can you seriously afford to take 3 months of your salary, withdraw it as cash, and set it on fire? I don't know anyone who would do anything that stupid; if I'm going to lose 3 months' pay, I might as well just take a 3-month unpaid vacation instead of working my ass off for money when then gets wasted.
And no, a house downpayment is not a "needless extravagance"; that is an utterly stupid thing to claim. People need a place to live, and while you can debate things like rent vs. buy, real estate appreciation rates, and the current market conditions, in general, for middle-class people, home ownership in the US is a fairly stable store of value if you don't plan to move for a while, and owning your own home means you don't have to pay rent any more. If you don't like the mortgage example, change it to something else, like putting the money in the bank or a stable value fund in case you lose your job. Or into a good index fund, where it's most likely going to appreciate significantly over the years. There's plenty of places where you can invest 3 months' pay that are perfectly sensible and will give you a far better return than just lighting it on fire.