"No it's not" and I absolutely agree with that. But can a "yes" also be a "no"?

A few months ago I had a discussion here about the extent to which it is considered rape for a woman to have unenthusiastic sex with her husband, even if there is consent, even if there is no threat or coercion. I'm confused by this discussion because some people said that a woman needs to answer "yes" enthusiastically, otherwise it's not true consent. Furthermore, several factors should be analyzed to understand whether this “yes” was in fact true. But if the wife says yes and the husband questions, it ruins the mood and causes an awkward interrogation. How can a man be sure that his wife/girlfriend is consenting?