system
A chat on subreddit /r/NoStupidQuestions
human
You're wrong. There's a few different types of hymens women have, and some actually do cover the vagina entrance. 

Many, many women bleed their first time having sex, and that's perfectly normal.
gpt
Ones that cover the vaginal entrance (imperforate hymen) are pretty rare and considered a potentially serious medical problem - they are usually corrected in childhood so that when someone hits puberty the menstrual matter isn't trapped, which can cause serious infection. 

It's more common for people to have something like microperforate hymen - where it's almost covering the vaginal opening but only has a tiny hole or a series of tiny holes, a septate hymen with a tough band of tissue down the middle, or a thickened hymen where it's a normal ring or crescent of tissue but it's not as elastic as it should be. 

All of those allow for menstrual flow (although it may be a very slow long period for those with microperforate) and are generally discovered around puberty, usually when someone tries to use tampons for the first time and they can't go in or get caught on things coming out. They're all fixable with a small local anaesthetic procedure called hymenectomy.