Why do women wear thumb ring?

Answer 1

Because it is fashionable and the woman likes it. Don't read any more into it than that

In the old days a woman wearing a ring on the left thumb meant she is married ,but available for sex and that is still the same today

Answer 2

There is not necessarily a perfect answer to this question. [This is defined by a large context of sociology... and there is no definitive answer to the socioligical reasoning as to why a person dresses a certain way.] How a person dresses the way they dress is dependent on a multitude of factors: religious ideology, community, social factors, recreational activities, events, the clothing types available to them, status symbols within groups or sub-groups, personal preference, (certain items are plausibly a symbol of status within certain sub-groups of members within certain sexual orientations, but simply because anyone wears anything as a status symbol doesn't necessarily mean that it is universally accepted by all groups within that category...) and so forth.