Because it's an outdated truth.
In a world where little girls didn't run around when they played, or rode bikes or horses, their hymens were still intact at first sexual intercourse, so it tore and they bled.
In the modern world, that doesn't happen often ... and there are actually some sick practices, like tying a young girl's legs together, to ensure that her hymen stays intact so she'll bleed.
Show him this information, and if he still thinks you're lying, dump him right off! That's disrespectful.
I wondered about this too though because I beleived OMG it was going to hurt soo soo bad and all that jazz.. no not AT ALL it was pretty fun lol
Now I will make sure that other girls dont have to beleive that like I did even though no one beleived I was lying
That is jacked up that he does not beleive you and that was a precious gift you gave him and he should be appreciative of that gift!