If a man and a woman want to reproduce sexually, they have to engage in sexual intercourse. This involves placing the man's erect penis into the woman's vagina. The vagina rubs and puts pressure on the penis, which makes the man have an orgasm. During his orgasm the man ejaculates, and his semen flows into the woman. If a sperm in the semen reaches the woman's Fallopian tubes and fuses with an ovum, this causes the woman to become pregnant. A man and woman can also have sexual intercourse for pleasure, without wishing to have a baby. Men and women, and gay couples (two women or two men) can also use their sex organs in other ways for pleasure.