What are the Sharia limits on sexual relations between spouses when they disagree, taking into account the privacy of this matter?

A wife is not permitted to refuse her husband's bed when he calls her, unless there is a legitimate Shar'i (Islamic legal) excuse. This is based on the Prophet, peace and blessings be upon him, who said: "If a man calls his wife to his bed and she refuses to come to him, and he spends the night angry with her, the angels curse her until she wakes up." And his saying: "If a man calls his wife for his need, she must come to him, even if she is at the oven." She must obey him and readily respond to him whenever he calls her, unless she has an excuse such as illness. And: "By Him in Whose Hand is my soul, there is no man who calls his wife to his bed and she refuses him, except that the One in heaven is displeased with her until he (the husband) is pleased with her." Both spouses should be patient, enduring, and seek excuses for one another.