Well in my humble opinion, there is nothing wrong if a woman marries a younger boy. And there is no law which states that its wrong even.
In Indian context, we have a tradition of arranged marriage and it is generally thought that if husband is elder, he has better understanding and knowledge of world outside. An elder man can control things around him better. Where as if the wife is elder, she may take a dominant position in the house and it may result in shift from Patriarchal system to matriarchial system, which was not actually acceptable to many hindus a few hundred years back. And this is the reason why, they searched for girls who are younger than boys and hence it became a tradition.
The current scenario has completely changed the mind sets of almost every youngster. People now don't take things like caste, color, age as criteria for marriage but yet there is a large section of population who sticks to the age old traditions.
Marrying a guy younger is not wrong, but our society is not accepting it. And anything which is not acceptable to mass is not right as far as everyone sees it.
Regards
Magician