In 1984, it is forbidden to love anyone but the Party. Family relationships, including those between husband and wife, ought to be secondary to personal allegiance to Big Brother. It is highly encouraged, for instance, to make oneself a first line of defense for thought crimes within a family. If a woman's husband is being suspicious, it becomes her duty to report him. Protecting him, out of a greater allegiance to her husband than to the Party, would be criminal, too.
Sex is considered a duty to the Party. It ought to be slightly unpleasant but done out of a sense of obligation. If sex were anything besides this, it could undermine the disconnection within family units. Sex cannot be forbidden, because the reproductive value is necessary, but Big Brother would like to make sure that that's the only purpose it can ever fill.