Men who know better don't tell a woman her foul moods must be inspired by pre-menstrual syndrome or PMS. The reasons aren't as obvious as one might think. I once told my father that if he had abdominal cramps, a headache, couldn't fit into his pants and was bleeding from his pee-hole that he'd be irritable too.
This isn't quite fair and circumvents a real understanding of why attributing disagreements to hormones is upsetting. While physical discomfort can shorten one's patience the fact remains: whether the source of the upset is real or perceived the emotional response is definitely real [or you wouldn't be trying to explain it away]. It's important to a woman that her partner treats her emotions with care and respect. Attributing her emotions to PMS means that you've made a dismissive personal attack instead of addressing the problem.
In a society where men are often socialized not to acknowledge their emotions, it might seem easier to tell your partner why she shouldn't acknowledge them either. You may come to realize that you've actually made things more difficult when she gets upset that you've invalidated her feelings and indeed a part of her. Now you're fighting about that instead of the 'real' issue. Seriously, this could happen:
Chick: Ow, you stepped on my foot.
Dude: I didn't step on it that hard. You're just sensitive because it's 'that time of month.'
Chick: *kicks dude in the nuts*
Chick: I didn't kick you in the nuts that hard. You're just sensitive because you masturbate so much. Besides, now you've hurt both of my feet.
Anyways, happy relationship advice day!