All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Among the required etiquettes when relieving oneself is to conceal oneself and be out of the view of the people as confirmed in the guidance (Sunnah) of the Prophet .
Al-Kaafi, a Hanbali book authored by Ibn Qudaamah reads: “If one is in an open space, then he should go as far away (from others) as he can as Jaabir narrated: “When the Prophet would want to defecate, he would go away until no one was able to see him.” He should also conceal himself from the eyes of the people as Abu Hurayrah narrated that the Prophet said: “Whoever wants to defecate, he should conceal himself and if he does not find anything to conceal himself with, he should make a pile of sand and turn his back to it.”
Indeed, the spouses are the first people who should act according to these etiquettes as a way of following the guidance of the Prophet because breaching these etiquettes may lead to aversion and hatred between the spouses, and this may lead to separation. Therefore, the spouses should pay attention to this and avoid what could lead to separation.
However, if your intention is just to ask whether the Sharee’ah permits it or forbids it, and whether one is sinful or not sinful if he does it, then regardless of the issue of the etiquettes and good conduct regarding doing it, it takes the ruling of looking at or touching the ‘Awrah (parts of the body that should be covered in Islam) of other people and it is well known that all this is permissible between the spouses.
Ibn Qudaamah said in his book entitled Al-Mughni: “It is permissible for either spouse to look at the entire body of his/her partner and to touch it, even if it is the private parts as Bahz Ibn Hakeem narrated from his father that his grandfather said: “O Messenger of Allaah, what are we allowed to uncover of our ‘Awrah and what must we conceal? He [the Prophet ] said: ''Protect your ‘Awrah except from your wife or those whom your right hand possesses (i.e. slave women).” [At-Tirmithi, who graded it as Hasan (good) Hadeeth] [End of quote]
Nevertheless, saying that this act is permissible for the spouses and that there is no sin on them if they do it, does not mean that a person may do it without a necessity. Indeed, good moral conduct is a part of religion and good morals are the foundation of nations as a poet said:
Indeed, nations are strong with their morals, and if their morals are gone, they will follow (perish).
In fact, many Prophetic narrations confirmed this. For example, the Prophet said: ''I was sent to complete good morals.” [Maalik]
Also, the Prophet said: ''Shall I not tell you about the most beloved person to me and the closest to me on the Day of Resurrection? [and he repeated this three times or twice]. They (the Companions) said: “Yes, O Messenger of Allaah.” Thereupon, he said: ''It is the one who has the best moral conduct amongst you.” [Ahmad]
Allaah Knows best.