There is nothing wrong in a husband thinking sexually of the wife, and also there is nothing wrong in a wife thinking sexually of her husband, as the issue is originally permissible. But there are certain guidelines that govern the whole issue in order to protect the Muslim husband and wife from succumbing to satanic whispers and hence committing something haram.
Here, we would like to cite for you the following fatwa:
“It is permissible for each spouse to think sexually of the other, but the following main points should be observed:
1-The Muslim should not keep away from his wife for more than six months, as stipulated by the Caliph `Umar Ibn Al-Khattab (may Allah be pleased with him). Exegetes of the Glorious Qur’an narrate the following incident in support of this opinion.
One night Caliph `Umar Ibn Al-Khattab (may Allah be pleased with him) was making his rounds of Madinah when he heard a woman singing:
The night is long, darkness all around me.
I am sleepless, for I have no friend to play with.
By Allah, had there been no fear of Him,
This cot would be shaking from side to side.
Upon investigation, `Umar (may Allah be pleased with him) found that the woman’s husband had been on a military expedition for a long time. He then asked his daughter Hafsah, a widow of the Prophet (peace and blessings be upon him), “How long can a woman endure separation from her husband?” She replied, “Six months.” As a consequence, he decided that he would not send a married man away from his wife for a period exceeding six months.
If a Muslim is away from his wife for a period longer than that, then both parties may fall into temptation and be affected by whispers from Satan.
This may lead them to think about haram things, and after thinking about such things he/she might be induced to satisfy his/her desires by doing something haram.
2-The Muslim who has no means but to stay away from his wife should break the power of his desire by fasting, lowering his gaze, avoiding temptation, and turning away from it. He must fear Allah as Allah says, “…and follow not the footsteps of Satan. Verily, he is to you an open enemy…” (Al-Baqarah 168)
3-Another issue that has to do with the question in point is that it is not permissible for a woman to describe another non-mahram woman to her husband in such a way that he would imagine her as if he is looking at her.
It was narrated that Ibn Mas`ud (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: “No woman should look at or touch another woman to describe her to her husband as if he can see her.” (Narrated by al-Bukhari, 4942).