The evolutionary logic of the female orgasm is pretty obvious. They have the same muscle spams men do only inward bound, thus if the timing is right they move the sperm of men who are more "skilled" closer to the egg giving them an advantage over less "desirable" men.
Plus of course there's the most basic reason for both: fun. How many people really and truly have sex for the sole purpose of breeding? How much of the perpetuation of the human race is because the process of making a kid is fun?
That ideal "timing" requires the man to ejaculate before the woman achieves orgasm and then to continue until she does shortly thereafter.
That's a tight window to work in...so to speak.
I was waiting for someone to state the obvious. Sex is by design pleasurable for humans so it compells us to do it, and that furthers the species. Animals have instincts, that lead to copulation when the female is in heat, we don't. Instead of the instinct we get 24/7 sex drives.