I see tnt666 so one can only be reasonable if they agree with you - interesting viewpoint.
The reason that doctors will not go on record and say that a woman / girl is a virgin depending on the presence of a hymen is because they can be torn earlier in life - so she could be a virgin but not have a hymen or yes you can be born without one (approximately 10%) strangely it's about the same percentage of women who are born without a G-spot.
Here are a few internet sources for you.
The Anatomy of the Hymen
Hymen repair on the Arabic Internet
The Psychology of the Hymen
The Appearance of the Hymen in Newborn Girls
Healthy Strokes
Not alot of Info but illustrations of the different types of hymen
The Mysterious Hymen Revealed
Some of them are abstracts I know - sorry about that, as a hospital employee I have subscriptions paid for but alot of them won't allow for download and the Hospital IT crowd checks IP addresses - I'll check at work or the libraries and see if I can find them as a hard copy
As for why the Hymen isn't broken - in alot of Asian countries sex toys are actually illegal (Indonesia and Japan for example) so the women in those countries are unable to break their own, those in Muslim countries will not because if they do then they are un-able to be married, it's a matter of their and their families honour that the hymen is intact on their marriage night.
In Western countries alot of women do break their own hymens. Unfortunately that might not occur to a girl until after it has been broken via phsyical activity or in the case of sexual abuse isn't an option.