There are numerous competing theories as to why this happens. Some suggest that the wedding ring is a cue that a man is "safe," a passing opportunity for empty flirting; while others theorize that the female psyche sees the ring as an indication that another woman has deemed him worthy. Perhaps practice is the key to being a good mate, and females tend pick the male they know has put in some hours. Maybe it's because people generally want what they can't have - you know, forbidden fruit is sweeter.
This kind of behavior is also observed in several species of fish and birds — and especially in guppies, which is one of the most popular freshwater aquarium fish species in the world. In the animal world, this is known as as guppy syndrome or "mate-choice copying." It was first documented in 1996 that female guppies, which normally prefer to mate with large and colorful male guppies, are easily persuaded to pick a smaller and less colorful male if they observe another female mating with him. Since it's hard to get into the psychology of a guppy, we have to project a human interpretation on what we see: the females assume there's something special — if subtly so — about males who've already been chosen as mates.
So the question is, is the wedding ring effect a human-level analogue to the mate-choice copying we can observe in the animal kingdom? Is it possible to extrapolate this behavior to ourselves? Has anyone experienced the guppy effect/wedding ring effect firsthand?