It's not about keeping it fair that even men should wear some form of accessory indicating they are married.
Wearing a mangal-sutra is a tradition similar to a sister tying a rakhee on her brother's wrist to remind him of his duty to protect her.
Even in that case, there is no equivalent of rakhee that a sister has to wear.
I hope that answers the question.
hinduism.stackexchange.com/users/151/Vikram Deshmukh
First a small clarification from Wikipedia entry Contrary to false impression generated in recent decades by Indian movies and TV soap opera, the practice does not exist in every part of India, nor is it an integral part of a marriage ceremony.
Apart from the mangala sutra, the toe rings (bichhua), kumkum, bangles, nalla pusalu (black pearls) and nose ring form six symbols that may indicate that a woman is married.
So when we say Mangal Sutra, we mean any of the above symbols that indicate marriage status.
Coming to the question, I think it has to do with prevalence of Polygyny .
A man could have many wives, but not otherwise.
So women would need an indication for their marital status, a "taken" tag if we may call it.
Where as men don't need any.
As I said this is just my guess, have no references.
Edit:
Mangala Sutra is integral part of Hindu Marriage.
This practice is an integral part of a marriage ceremony as prescribed by Manusmriti the traditional law governing Hindu marriage.
Polyandry also existed in Hinduism but it was less prevalent when compared to Polygyny.
Reference:
Polygamy:
A Cross-Cultural Analysis.
Berg Publishers.
p. 109.
hinduism.stackexchange.com/users/23/Tejesh Alimilli
The Mangal-sutra can be a symbol of monogamous human pair-binding.
Early Hindu society was a matriarchy where children were referred using their mother's name.
For example, son of Radhya was called Radheya, son of Krithika was called Karthikeya.
This was because, in many cases polygamy was prevalent and a women didn't really know who the father of the child was.
One could be sure who the mother was as she gave birth to the child.
When monogamous relationships evolved, the men had to take responsibility for the children they fathered.
This created a sense of ownership.
When men tie the mangal sutra, it instilled in them a sense of duty.
It also made it clear to other men that the woman is another man's wife and he shouldn't mate with her.
This answers why only woman wear a symbol signifying that they are married.
There is also a story that all girl children were considered the property of devas as soon as they were born and men had to symbolically win them and hence tie the mangal sutra.
This blog series on "Brief History of Hindu Marriage" explains many rituals surrounding marriage.
hinduism.stackexchange.com/users/14/Bharat
Things like 'Mangalsutra' and 'Sindoor', worn only by married women, must have started out as a way for women to feel safe (signifying that they are not 'available') and later became a mandatory ritual over time.
Then it makes some sense why such things never happened for men.
Also, like all parts of the world, indian society was (is partly even now)
acutely male dominated .
There are many rituals which were biased and unfair to women like 'Sati'.
This male dominance would have resulted in many simple rituals turning into torture for women.
Like in case of 'Mangalsutra' and 'Sindoor', stripping them off a woman while she's grieving for her husband's death.
hinduism.stackexchange.com/users/96/Swapnil Luktuke
Men too have some symbols but it varies according to customs.
In brahmins I have watched some sampradayas where brahmacharis wear dhoti without kacche and after marrying they wear dhotis with kacche.
I have also watched they start to wear two or more yajnopaveetas after marriage.
hinduism.stackexchange.com/users/438/Ramaprakasha
I think because married women have to always look good and as they liked to be well groomed for their husband they do wear these symbols.
In past Bindi (tilak) and earrings were mandatory for men too but as the civilisations developed women's eagerness to always look well groomed increased and in men it deceased.
Thats why women wear so many things: mangalsutra, sindoor, bindi, earrings, colourful bagles and toe rings etc.
and slowly made them a standard for a married women.
hinduism.stackexchange.com/users/3571/Ashu Singh
Mangal sutra is one of the seven symbols that indicate a woman is married, whereas married men have only one (a toe ring).
These symbols are stripped away when the husband dies
It is not mandatory for the bride to wear the mangal sutra either, only the kumkum on the forehead is.
hinduism.stackexchange.com/users/160/amarprabhu
In Hinduism, brides wear Mangal Sutra to signify that they are married. Additionally, the use of sindoor, red kumkum powder also applied along the parting of the hair, is also worn by married women. These are all symbols that signify that a woman is married.  Why don't such symbols exist for men?
Why don't men wear the likes of Mangal-Sutra to signify they are married?
hinduism.stackexchange.com
<marriage><women><symbolism><gender>
hinduism.stackexchange.com/questions/125
hinduism.stackexchange.com/users/75/user3459110
Mangal sutra is one of the seven symbols that indicate a woman is married, whereas married men have only one (a toe ring).  Brahmacharis wear dhoti without kacche and after marrying they wear dhotis with kacche. It is thought that the absence of a common lack of symbol of marriage for men is due to the prevalence of polygyny. There are some symbols of marriage for men, however - dhotis with kacche, two or more yajnopaveetas or a toe ring, for example.
It is thought that the absence of a common lack of symbol of marriage for men is due to the prevalence of polygyny. There are some symbols of marriage for men - dhotis with kacche, two or more yajnopaveetas or a toe ring.