Yes it is permissible for each spouse to think of the other, but we must explain the following points with regard to this matter:
1 – The Muslim should not keep away from his wife for more than six months, as stipulated by Ameer al-Mu’mineen ‘Umar ibn al-Khattaab (may Allaah be pleased with him), as was narrated by ‘Abd al-Razzaaq in his Musannaf, 7/152.
If a Muslim is away for longer than that, then both parties may fall into temptation and be affected by whispers from the Shaytaan.
This may lead to thinking about haraam things, and after thinking about such things he may be provoked into satisfying his desires, and this may lead him to falling into haraam – Allaah forbid. Desire has power over a person’s mind and this may lead him to look at pictures or haraam things.
2 – The Muslim must break the power of his desire by fasting, lowering his gaze, avoiding temptation and turning away from it. He must fear Allaah as Allaah says (interpretation of the meaning):
“and follow not the footsteps of Shaytaan (Satan). Verily, he is to you an open enemy”
3 – Another issue that has to do with this topic is that it is not permissible for a woman to describe another (non-mahram) woman to her husband, so that he will not imagine her as if he is looking at her.
It was narrated that Ibn Mas’ood (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah 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-Bukhaari, 4942).
And Allaah knows best.