Why Lord Shiva is treated as ideal husband material by Indian women. Why Every unmarried Girls dream to have a Husband like Mahadeva for an example in our area they do 16 Somvaar Vrat, Shivratri Vrat etc and pray Lord Shiva as husband or to get someone having the qualities of Lord Shiva?
This is quite a speculative question. I'm not sure if scriptures do mention a very specific reason as to why un-married females observe Monday fasts or pray to Bhagvan Mahadev Shiv ji. Maybe it's to get a husband like Shiv (which seems impossible, because only Shiv ji can be like Shiv ji, no one else), or maybe to get a good, righteous and pious husband, which seems more correct. Shiv ji is popularly thought of to be easily pleased compared to other deities. Hence called Ashutosh and Bholenath. Maybe that's a reason women worship him.
Also, in Shaiva, he is uniquely monogamous with one only wife, his other literal Half - Shakti, while Vishnu ji represented mostly with three wives (as per Brihat Parashar Hora Shastra, Ch. 1 - Shri, Bhu and Nila while Devi Bhagvat says Lakshmi, Sarsvati, Ganga, and, even Brahmaa ji is said to have two wives Gayatri or Saraswati and Savitri, varying as per purans)
Another thing is that as per Shiv Puran there's no chanting restriction as regards Shivling's worship using the Panchaksharii Mantra = नमः शिवाय, (Can a Sudra become a Brahmin in 'this' birth?) Which is quite unlike other mantras like OM Namo Bhagvate Vasudevay, Om Namo Narayanay, etc which require strict laws of initiation (Can anyone chant Om Namo Narayana?) from a right guru and other strict observances and requirements like being a dvij by birth, which women might be forbidden from (https://hinduism.stackexchange.com/a/24231/30105), as per the traditional orthodox sects. In case of Shiv jis Worship especially using Panchaksharii Matra and Shivling worship it seems open to all mostly and even chandalas are not forbidden the Panchaksharii mantra, let alone uninitiated women. Even the name Shiv recitation seems to have great benefits making one fit for worship etc (What Shruti source is SriKanthacharya quoting in this bhasya regarding efficacy of name 'Shiva'?).
Now, we also have another explanation from Vaishnav perspective on this.
Srila Prabhupad swami of the gaudiy vaishnav sect, in a purport to Shrimad Bhagavat Puran gives the reason as follows
Purport by Prabhpad ji on Shrimad Bhagvatam 3.23.1
The specific example of Bhavānī is very significant. Bhavānī means the wife of Bhava, or Lord Śiva. Bhavānī, or Pārvatī, the daughter of the King of the Himālayas, selected Lord Śiva, who appears to be just like a beggar, as her husband. In spite of her being a princess, she undertook all kinds of tribulations to associate with Lord Śiva, who did not even have a house, but was sitting underneath the trees and passing his time in meditation. Although Bhavānī was the daughter of a very great king, she used to serve Lord Śiva just like a poor woman. Similarly, Devahūti was the daughter of an emperor, Svāyambhuva Manu, yet she preferred to accept Kardama Muni as her husband. She served him with great love and affection, and she knew how to please him. Therefore she is designated here as sādhvī, which means “a chaste, faithful wife.” Her rare example is the ideal of Vedic civilization. Every woman is expected to be as good and chaste as Devahūti or Bhavānī. Today in Hindu society, unmarried girls are still taught to worship Lord Śiva with the idea that they may get husbands like him. Lord Śiva is the ideal husband, not in the sense of riches or sense gratification, but because he is the greatest of all devotees. Vaiṣṇavānāṁ yathā śambhuḥ : Śambhu, or Lord Śiva, is the ideal Vaiṣṇava. He constantly meditates upon Lord Rāma and chants Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare. Lord Śiva has a Vaiṣṇava sampradāya, which is called the Viṣṇusvāmī sampradāya. Unmarried girls worship Lord Śiva so that they can expect a husband who is as good a Vaiṣṇava as he. The girls are not taught to select a husband who is very rich or very opulent for material sense gratification; rather, if a girl is fortunate enough to get a husband as good as Lord Śiva in devotional service, then her life becomes perfect. The wife is dependent on the husband, and if the husband is a Vaiṣṇava, then naturally she shares the devotional service of the husband because she renders him service. This reciprocation of service and love between husband and wife is the ideal of a householder’s life.
Thus we might conclude that every unmarried girl wants a husband like Shiv ji because
Shaiva considers him to be the Param Purush, thus the ideal for a husband
Shiv-Shakti as perfect equal half in the Bhagvan Ardhnarishvar murti exemplifies the best ideal for a conjugal married couple, that is equal and devoted to each other both in and outwardly.
Shiv ji as the foremost Vaishnav represents the highest perfection a woman can want in her husband since, it is said that wife gets half the merit.