The male physiology at the level needed to understand for basic sex education is pretty simple.  It’s basically enough to understand that the testes are the male equivalent to the ovary, and produce both sperm and the male sex hormone testosterone.  The penis is flaccid in its resting state and becomes erect when it becomes engorged with blood due to sexual excitement.  The physiology of this complex, but this level of understanding is adequate.  During intercourse the male ejaculates semen, the fluid that contains sperm into the female vagina.  Sperm can live in semen for up to two days or slightly more.

The female physiology is helpful to understand in some detail.  Unlike the male who has a constant supply of sperm, the female only passes one egg (usually, occasionally passes two or more) each month.  To understand the female monthly cycle it’s easiest to first understand the ovarian cycle.  Each month that the prior month’s egg does not become fertilized and implanted to start a pregnancy in the ovaries several eggs start to mature.  One of these becomes dominant, and progresses to a fully developed egg.  By about 14 -18 days after the start of the prior menses the mature egg is released.  This is called ovulation.  The ovulated egg moves through the fallopian tube toward the uterus.  If the egg is fertilized by a sperm it may implant in the lining of the uterus and a pregnancy develops.  The developing pregnancy produces hormonal feedback to the ovary to continue to produce progesterone to support the developing pregnancy until the placenta can assume that function.  If no pregnancy occurs the ovary continues to produce hormones only for about 14 days until there is not hormonal feedback from the developing embryo and the ovary stops producing the progesterone hormone needed to support the growth in the lining of the uterus.   This leads to the lining of the uterus being sloughed off as the menstrual cycle.  The bleeding from the shedding of the uterine lining typically lasts for 3-8 days and is known as the menses, or menstrual period.