I don't know where you live but this sounds absolutely outrageous!!!! Don't parents have an influence in this decision?
Besides, you can loose your hymen which is a way of seeing if a girl is sexually active by simply riding a bike so I don't know what they are talking about.
Don't do it as you have the right and it is your body or get your parents to fight for you.
I live in the USA and that would be totally illegal for them to do.
I hope your question is valid. This question has gotten everyone riled up.