Several females curb to sexual pressure regardless of the amount of pleasure that they actually get from the act, though. While the psychological reasons and reinforcement in having lots of sex are important, there is also a lot of important (and kinda really fascinating) physical processes happening that increase the chance of fertilization.

Approaching orgasm, a woman's cervix dialates, which facilitates the passage of sperm into the uterus. (Don't forget the "purpose" of sex is having babies.. or something..) Also, the shape of the vaginal cavity actually changes slightly to make deeper (again, get into that uterus!) penetration possible. Finally, during orgasm the muscles surrounding the cervix spasm; the contractions force the cervical opening down into the semen that's hopefully been ejaculated nearby.

There is also the matter of NumOfOrgasms: that women can more readily have more than one. The cervical spaz isn't going to do much good without any cum around, so the female orgasm must take place later in the coital process. The possibility of many orgasms increases the likelihood that they will coincide with the presence of semen. Multiple orgasms will also prolong the period of the cervical exposure to the sperm pool. (Although the cervix doesn't really dip down to the bottom of the vagina!^!)-->

Fortunately, techonology is high enough that sophisticated techniques of preventing these evolved traits from doing their jobs have been developed.

That way, the "reason" for female orgasm is fun... or anything you want it to be.