This is traditionally what is thought of as intercourse. In the female virgin the entrance to the vagina is often partially blocked by a membranous tissue called the hymen. The size of the hymen varies greatly among females. Some virginal girls have virtually no hymen and others have enough tissue to make using tampons difficult. The presence of the hymen will sometimes make the first vaginal intercourse painful and bleeding is not unusual.
Subsequent intercourse is generally much easier.
Orgasm is the rhythmic contraction of pelvic muscles and pleasant sensations that occur after a buildup of sexual tension and arousal. It is caused by a combination of responses of the autonomic nervous system and to a hormone called oxytocin. In men orgasm is accompanied by ejaculation. Both men and women are able to have orgasms but reaching orgasm requires a longer period of sexual arousal in the female. Arousal in both sexes is manifested by an increase in blood flow to the genitalia resulting in an erect penis or swollen labia and lubricated vagina.