I've heard of royals and high-status nobles exchanging jewelry before and during marriage ceremonies very early on in the medieval period. However, I assume that in the medieval period the middle class (such that it was, consisting of lesser knights and skilled artisans and whatnot) would not have been able to afford enough gold to waste much on wedding bands. But sometime in history, gold wedding bands became more or less the "default" matrimonial jewelry for middle class people in Western culture. When did this happen, why did this happen, and where did all the gold to support this wedding ring habit come from?
Wedding rings and gold rings in general existed and were prevalent way before the Middle Ages. Pliny the Elder wrote in Natural History that Emperor Severus extended the right to wear gold finger rings first to Roman soldiers and then to all free citizens (before that only ambassadors and senators could wear gold rings), so affordability has never been an issue. The Encyclopedia Britannica states: "The giving of a ring to mark a betrothal was an old Roman custom. The ring was probably a mere pledge, pignus, that the contract would be fulfilled. In Pliny’s time [about 70 A. D.] conservative custom still required a plain ring of iron, but the gold ring was introduced in the course of the second century. This use of the ring, which was thus of purely secular origin, received ecclesiastical sanction, and formulae of benediction of the ring exist from the eleventh century." As Roman citizens grew wealthier more and more people started wearing gold rings. Early Christian writer Tertullian condemned Roman citizens for wearing too much gold jewelry except for wedding rings. Clement of Alexandria wrote: "The Word of Christ permits them [women] a finger-ring of gold. Nor is this for ornament, but for sealing things which are worth keeping safe in the house, in the exercise of their charge of housekeeping."
There were plenty of economic lean times thereafter, but by definition "middle class" (a social class that didn't grow very large until the Industrial Revolution) should be able to have some money for luxury. I'm not sure what you mean by "afford enough gold to waste much on wedding bands" as for most people throughout history wedding is the most important event of their lives, they would at least get a wedding ring and a wedding ceremony if that's the only luxury they were to pay for. Some religious orders forbid any kind of jewelry including the Puritans who use thimbles instead of rings; many Mennonites still forbid jewelry; or John Wesley who wrote "This is no time to give encouragement to superfluity of apparel...Allow no exempt case, not even of a married woman. . . . Give no admission to those who wear rings." (However jewelry has not been an issue since the 1850's and in 1872 manual of the Methodist Church states "If the parties desire it, the man shall here hand a ring to the minister, who shall return it to him and direct him to place it on the third finger of the woman’s left hand.")
I know rings have been used widely among both Jews and Christians of Europe for quite a while. One interesting artifact in this tradition is the use of ceremonial community wedding Rings, like this one or this one. These large, ornate rings were owned by the community, and were used and worn only during the wedding ceremnony and celebration -- sometimes a more modest, practical ring, would be gifted to the bride for everyday wear, as part of the wedding gifts. I know that Jewish communities in Western and Central Europe persisted in this practice right up until the early 19th century. I think the practice has medival origins, but I've never actually seen a "community ring" or a reference to one from before the 14th century, but that might be just a reflection of what artifacts and records have survived.
I don't know if this was an exclusively Jewish practice at the time, but I wouldn't be surprised it had a contemporary Christian analogue.
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