Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- There is no difference between men and women when it comes to ghusl after sexual intercourse. Neither of them have to undo their hair for ghusl. It is sufficient to pour three handfuls of water over their hair and then to pour water over the rest of their bodies.
2- If the man or woman has sidr, dye or something of that nature in his hair that would keep the water from reaching to the roots of the hair, he or she must remove that.
Elaborating more on this, the Standing Committee for Scholarly Research and Iftaa’ in Saudi Arabia, states:
There is no difference between men and women when it comes to ghusl after sexual intercourse. Neither of them have to undo their hair for ghusl. It is sufficient to pour three handfuls of water over their hair and then to pour water over the rest of their bodies.
This is based on the hadith of Umm Salamah who said:
“O Messenger of Allah, I am a woman who has closely plaited hair on her head, should I undo it for making ghusl from sexual intercourse?” The Prophet (peace be upon him) told her, “No, it is sufficient for you to throw three handfuls of water on your head and then pour water over yourself, and you will be purified.” (Muslim)
However, if the man or woman has sidr, dye or something of that nature in his hair that would keep the water from reaching to the roots of the hair, he or she must remove that. If it is something light and does not keep the water from reaching the roots, it does not have to be removed.
As for the woman making ghusl after her menses, there is a difference of opinion over whether she must undo her hair or not.
The correct opinion is that it is not obligatory upon her to do so. This is based on the narration of Umm Salamah mentioned above.
This narration is a clear text that it is not obligatory upon the woman to undo her plaits after either menses or sexual intercourse.
However, it is best for her to undo her hair while making ghusl after menses as a precautionary measure, as a means of avoiding the thing in which there is a difference of opinion and as a way of reconciliation of the different evidences.
Almighty Allah knows best.
Source: Islamic Fatawa Regarding Women, Compiled by Muhammad Ibn Abdul-Aziz Al-Musnad and translated by Jamal Al-Din Zarabozo, Darussalam, 1996.