A medical study done (sorry can't remember the details) found that only 1 in every 5 baby girls have hymens. The modern girl uses tampons, rides bikes and horses, run around, climb trees, and therefore it is very easy for her to stretch her hymen. I am using a menstrual cup, which is worn like a tampon, but it is much bigger than a tampon, and the producers actually warn that it can stretch and tear the hymen when it is removed.
And some have vibrators/dildo's shaped like a real penis. If you, someone else or herself plays or masturbates with her it can also cause the hymen to tear, it depends on her own anatomy. It further depends on penis size, the size of her own vagina, and how relaxed she is when you penetrate her.
If you say "blockade" what do you imply? Stiffness? Something physical? If a woman has been celibate for a long time, and she is suddenly penetrated again, the man can assume she is a virgin because she feels like one- she is stiff inside.
What is a virgin? If she had sex with a girl before, but not with a man, physically she can still have a hymen, but is still a virgin? How do you define sex- exclusively as penis-in-vagina? What is a virgin today? Very abstract concepts!
Please don't think if she did not bleed or if you penetrated her easily that she is not a virgin. That would be very unfair. Not all women bleed or are "blocked" the first time.
The Merck Manual of Medicine (Home Edition)
Google "virginity" for useful sites