There's actually a really good reason for that. The body doesn't have a good way to get rid of excess iron except by bleeding, so it's fairly easy for someone without a period to get iron poisoning from vitamins with iron in them. Women's vitamins assume the person taking them loses a significant quantity of blood every month. Not only should men not take them, women whose birth control eliminates their period completely shouldn't take them either.
This isnt entirely related, but your comment made me think about the time I went into CVS to buy multivitamins and noticed all of the "men's" included a picture of an orange while the "women's" did not. All the other fruit pictured were the same between the two, but not oranges.
I did that with buying "one-a-day" vitamins for seniors because they were a quarter the price of standard men's vitamins. I checked the stats and ingredients, they were about identical and from the same brand.
Earlier this year a doctor advised us (male and female) to take prenatal vitamins, and yesterday a nutritionist told us the same. They really just have everything anybody needs, apparently.