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
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.
It menas she is married ,but available for sex
The one on which the ring fits.
No.
I wear a thumb ring because I love the way it looks. That's all.
Because he is mentally handicap.
Fashion
In Indian culture, a girl may wear her promise ring on the ring finger of her right hand. The choice can vary depending on personal preference and cultural practices.
you wear it on your thumb
This means when you are not married just wear it for fun!
It means nothing
if she is wearing a ring on the left thumb means she is married but available for sex
People often say that if a man wears a ring on his thumb it makes him gay. No, having sex with other men makes you gay. A plain silver ring in my opinion can look good on some men's right thumb.