From Encyclopedia of Sex and Sexuality
There are two main body positions a couple may use during sexual intercourse: the couple may face each other, or the male may face the female from behind. In each position one or both of the partners might be lying down, sitting up in a bed, sitting in a chair, kneeling on the floor or bed, or standing. There is no one position that is “natural” or “normal,” nor is any position abnormal or a sign of perversion or pathology.
 Facing Each Other
The most common sexual position appears to be the so-called “missionary position,” in which the man is face down, above the woman (male-superior). The term was first used by indigenous people of the South Pacific to describe the preference of missionaries, who considered other positions sinful. The position is anatomically sound and usually pleasurable for both parties; it enables each partner to look at the other during intercourse and facilitates kissing and “love talk” between them. Psychologically, the male is sometimes seen as the aggressor in this position, and the female as the passive partner. This position may preclude sufficient stimulation of the clitoris while the male is thrusting his penis in and out of the vagina. Also, if the male is tired, he may lose his erection or feel uncomfortable supporting himself over the woman, or the woman may feel uncomfortable if the male is unusually heavy. On the other hand, some men find that the male-superior position is too stimulating and they ejaculate before they want to (a condition known as premature ejaculation). In addition, some women may feel more psychologically aggressive or assertive at times and want to remain “in control” of the sex act. Thus, many couples who experiment with a female-superior positon—the woman on top—find that it adds a new and preferred dimension to sexual variety.
Sex therapists report that many women who have experienced difficulty obtaining orgasm or who have partners who suffer from premature ejaculation can successfully utilize the female-superior position during treatment of either problem. The female-superior position seems to have most of the benefits of the male-superior position without its problems, and it has the enhanced benefit of making it easier for the male to caress his partner’s nipples or clitoris with his hands during intercourse. In addition many women report that it is far easier for them to reach orgasm, since they can move their bodies to maximize stimulation to the clitoris and continue this stimulation even if the male climaxes first. However, some men may feel uncomfortable having a partner in what they interpret as a dominant role.
Some couples may experiment with a side-to-side position, especially if they are tired. This position does not allow for deep penetration. Some may not find it satisfying, but others may use it during advanced stages of pregnancy or in cases where the length of the man’s penis may hurt his partner during deep penetration.
The rear-entry position involves penetration of the vagina by the penis while the man is behind the woman. Often the woman will kneel, supporting her body with her hands or elbows. While the rear-entry positon offers a different type of psychological and physical stimulation, it does not have any unique advantages over face-to-face positions except for its novelty. It does, however, provide deep penetration, desirable for some women, although others may find it uncomfortable.
None of the variety of positions that couples might envision should be rejected out of hand. Only after discussion and experimentation can sexual partners decide which positions are suitable and desirable for them.