They might minimize what’s happening (it’s just hormones, it’ll pass), which lands as dismissal. They might become defensive when the woman’s irritability lands on them, which shuts down any chance of real conversation. Or they might simply not know that perimenopause can begin a decade or more before the last period, meaning they don’t have a frame for understanding that what they’re witnessing is a transition with a physiological basis, not a personality change or a sign that something is fundamentally wrong with the relationship.