Its true that Islam asks the woman to seek husband’s consent for voluntary fasting. Holy Prophet (PBUH) said:

"A woman should not fast (optional fasts) except with her husband's permission if he is at home (staying with her)." (Sahih Bukhari, Hadith 4793)

To understand the logic behind this command we need to keep in mind few basic Islam principles.

1-Islam leaves no option for men and women to fulfill their desires except their spouses. Quran says about the husband wife relation:

“They are your garments and ye are their garments.” (Qur’an 2:187}

2-Islam makes woman the ‘Queen of the House’ and gives her the responsibility of house affairs while man is made the manager of out-of-house matters. It’s the man who is made responsible for financial matters and thus it is he who is to stay out of home and earn for his family.

So while out, he is much likely to come around any woman and this may cause some emotional upheaval within him. In such a situation Shariah has given a solution.

Holy Prophet (PBUH) said:

“When one of you sees a woman, he should come to his wife, for that will repel what he feels in his heart.” (Sahih Muslim, Hadith 2491)

This is a very natural and rational solution to any such situation. Now if a man faces such a situation and his wife is fasting then it will be a problem as intimate relations nullify the fast. And this can cause serious problems especially when it becomes frequent. It is fact that men have less power to control their desire. It is in only for this reason Holy Prophet (PBUH) gave such a commandment. This is only for voluntary (Nafal) fasts and only for conditions when the husband is staying with the wife. If woman is at her parents’ home or if the husband is away for some reason, she is not required to seek husband’s consent.

Such an order is not given for men as woman are generally not supposed to come across such a situation as they are ideally required to fulfill in-house responsibilities and thus stay at home. Thus there is much less chance of their coming across the similar situation.

Indeed Allah knows the best!