Updated: Sep 10, 2019
In the verse of Ḥijāb,
“They do not reveal their adornment except for their husbands.”
A husband can view more of his wife's adornment than others, as every area of her body is lawful for him to see and enjoy. Due to these implications, He began with the husband. This is because his span of permissibility with her is wider than others with her.
Allah, the Exalted, has stated,
“And those who safeguard their private parts except from their wives and what their right hands possess, for indeed, they are not to be blamed.”
Further, scholarship has differed on the permissibility of peering at the vagina of one's wife. There are two views here.
One of them states it to be permissible since if it is permissible to take pleasure in it then viewing it is of an even wider scope of permissibility.
Yet others state this to be impermissible due to the statement of ‘Ā'ishah, may Allah be pleased with her, in her mention of her circumstance with the Messenger of Allah ﷺ , "I neither saw that of him nor did he see that of me."
The first view is more preponderant, and the statement of ‘Ā'ishah is understood to be a matter of decorum and suavity. This has been stated by Ibn al-'Arabī.
Aṣbagh has mentioned, "From our scholars are those who have stated it is permissible for him to lick it with his tongue."
Ibn Khuwayzimandād has stated, "It is permissible for the husband to peer at the entire of the body of his wife. This includes the outside of the vagina but excludes the internal. As such, it is also permissible for the wife to peer at the private areas of her husband's body."
And in prophetic tradition, we cite the principle, "Women are the twins of men."
The meaning here is everything that applies to men also applies to women unless there is specific evidence to prove otherwise.
Source: Tafsīr al-Qurṭubī [12/232-233]
With summarization and adjustment.