Why was Ahalya cursed for her sin even though she was mentally pure?

[Indra came to Ahalya in the disguise of her husband, Sage Gowtama, and deceived her. But her husband cursed her for her unfaithfulness. Is it not injustice because in her mind she was only thinking of her husband? You say when the mind is not polluted the sin is not committed.]

Ahalya was the daughter of Lord Brahma. When Indra came to the abode of Lord Brahma, she saw him and loved him. But Lord Brahma and Sage Narada gave Ahalya to the Sage Gowtama in marriage. When Indra came in the disguise of Sage Gowtama, Ahalya identified Indra as said in the Valmiki Ramayana (Devarajam tu Vijnyaaya). Sage Gowtama knew everything by his super natural power and his curse was justified.

This entire story gives us a lesson that when a girl is married, her mind should be known. The parents should not force any girl for marrying a particular boy. Similarly the mind of Rukmini was on Krishna. But the parents and brother decided her marriage with Shishupala by force. The Manu Dharma Shastra says that the marriage is not completed by the promise (engagement), or by reciting sacred hymns in the marriage ceremony. The marriage is completed only when the marriage takes place by the willingness of the girl. Simply the engagement and the marriage function cannot complete the marriage (Vacha Datta Mantra Datta Mano Dattacha Kanyaka). Nowadays people give their girls in marriage as if they were cows, land or gold. This is wrong according to the Dharma Shastra. These two stories preach the same.

The story was only to preach the right knowledge. We should not think that Lord Brahma and Sage Narada were ignorant in giving Ahalya to Sage Gowtama. They enacted this drama only to preach the right knowledge. If Indra had enjoyed Ahalya in disguise, without her knowledge, Ahalya could not be cursed. Unless the mind involved, any karma [action] cannot bring the fruit. Thus when Sita was touched by Ravana, Sita was not polluted because her mind was not involved. But when Renuka saw Chitraratha on the bank of River Narmada, her mind was polluted [since she was attracted to him]. She did not even touch that fellow. But Parasurama cut off her head and she was punished. Therefore the sin is not in the deed but it is in the mind. The punishment is only for the mind and not for the deed. In the case of Ahalya, her mind as well as her body was polluted by Indra. Therefore Sage Gowtama cursed her and he should not be criticized.