What is the ruling on a man abstaining from marital relations with his wife without a valid excuse, and is he sinful for that, and what should the wife do in such a case?

Scholars differ regarding the obligatory amount of sexual intercourse for the husband. The most preponderant opinion is that it is obligatory for him to have intercourse according to his wife's need and his own ability. Ibn Taymiyyah's choice is that the obligatory intercourse should be commensurate with the wife's need and his ability, just as he feeds her according to her need and his ability, and this is the sounder of the two opinions.

If the husband refrains from having sexual intercourse with his wife without an excuse, despite her need, he is sinful and wrongs her by his ill-treatment and injustice. The wife should communicate with her husband to understand the reasons for his abstinence. If it is due to her shortcomings, she should rectify that by being a good wife and adorning herself. If there is no shortcoming on her part, she should frankly tell him that her chastity is her right, and that satisfying her need is part of good companionship for which the husband is rewarded, as the Prophet (peace and blessings be upon him) said: "And in the sexual intercourse of one of you there is charity."

If that does not help, then if the wife can be patient and maintain her chastity by fasting, guarding her limbs, and occupying her time with beneficial things, she should do so. Otherwise, it is her right to seek a divorce or khul'.