Because some men (not all) take your assertions that some men are unfeeling about your periods as an attack on all men.
Thing is, there are WOMEN out there who (for one reason or other) won't believe your periods are that bad - it's more an issue of people who don't experience bad periods than simply because of their gender. To a non-careful reader, it seems to make it about some attribute of all of men.
Unfortunately, the response of many of these men is to retort with the ways that they think men are (generically) mistreated or misunderstood.
And the entire thing escalates.
For instance, let's take one of my favorite ones: "Men will never understand the pain of childbirth".
Absolutely true. No guy will ever have childbirth so no guy will ever experience that exact pain (ignoring the fact that no woman experiences it exactly the same).
But men and women CAN both experience pain that is profoundly worse, e.g. large kidney stones. We know it's worse because women who have experienced botgh childbirth and large kidney stones have told us that kidney stones are worse.
So when a woman says that no man can ever experience the pain of childbirth, exactly what is her point? That men can't tolerate pain? That men can't understand the joy/wonder most women have when they hold the baby in their arms? That men can't push bowling balls through their naughty bits?
When you make a generic comment about the other gender with no qualifications, it's usually wrong on its face and likely to be offensive either to that gender or the other (non-binaries tend to be fairly philosophical as they've heard it all before. There are exceptions.)
Yes, lots of guys invalidate women's feelings. And visa versa. It's not just you, but as soon as you make it about a demographic, you're missing something important: People misunderstand each other.