Is the wife sinful for refusing to utter sexual words using terms for genitalia or common terms for intercourse, based on her husband's request, or is it permissible for her to refuse? And what is the ruling on the husband's refusal if the wife requests that of him?

It is permissible for spouses to utter the sexual words mentioned. The wife is not obligated to obey her husband in uttering sexual words if he commands her to do so, because it is not part of what is considered good and proper conduct (ma'ruf), and because it may harm her, and this is not part of conjugal relations based on ma'ruf. If the wife is not obligated to obey her husband in this matter, then the husband is also not obligated to respond to his wife if she requests it from him.