The hymen, which is a very thin piece of skin which partially covers the entrance to the vagina, may stretch or tear during masturbation, sport, or with the use of tampons.  Even though the hymen may be ruptured, unless you have actually had sexual intercourse, you are still a virgin.

A boy's first ejaculation usually occurs between the ages of 12 and 14, but sometimes earlier and sometimes later.  Each ejaculation contains about 5ml or 1 teaspoon of semen, and about 1 million sperm could fit on the head of a pin.  Boys, too, are virgins unless they have actually had intercourse.