Here’s what I understand about foreplay from the programming I’ve seen on channels like TLC and Science: women, biologically speaking, are interested in landing a man who will not only give her offspring, but also who will care for her and said offspring. A guy who is willing to invest time/effort in foreplay is more likely to be the guy who is willing to stick around should an offspring result.
Just my 2C
As for the orgasm part, as usual, WhyNot has it covered. 
There are other signs that are quite visible and not internal in a percentage of women.
You’re posting while enjoying a female orgasm? Dude.
I also understand that most sperm actually either tangle themselves up to block the sperm of other males, or are hunter-killers that seek out such “enemy” sperm and kill them.
I’ve also heard it speculated that another purpose of the female orgasm is to make cheating more efficient, since women are more likely to orgasm while cheating and orgasms increase fertility. Yes, women are interested in a man who will stick around to care for them and their children and not just men who are cute or seductive; but that doesn’t mean they have to be the same man.
Its very hard to type this way.
I can not let this brilliant post go by without calling it to everyone’s attention.
Remember that as far as evolutionary “design” is concerned, it just has to be good enough. Every aspect of a biological adaption has its plusses and minuses.
If men went off too easily, then they may not be in position in time. (Does that happen to you?) If you take too long, you’ll spend the next day fixing the ceiling plaster and painting the ceiling or remodelling the room. If it was too pleasurable then nothing else would ever get done.If it was half as pleasurable, then the ones with less pleasure would have less children… The build-up and orgasm both are plenty pleasurable enough to encourage a repeat performance in good time.
Plus, if it happened faster (and so more often?) then a lot of valuable biological resources would be wasted in non-effective reproduction activity. In the “good old days” of evolution, valuable resources like food were hard to find(??) and so using up a lot of stored energy was a survival minus.
If reproduction were as fast and easy as a quick handshake (hmmm…), the incentive to be selective (or the ability) might be much more removed from the reproductive equation, thus limiting the efectiveness of selection courtship rituals, etc. These determine the quality of the offspring based on the parents’ choosiness.
Similarly, I think the process for the woman is Mother Nature’s equivalent of “appropriate technology”. Too much pleasure, the mother would neglect the children to have sex. Too litle and she’d only want it once a year… (Oh. OK, I see.) Done right, it physiologically aids conception and also encourages a repeat performance.
You know they’re working on 2.0? I’m not on the design team but I was in one of the focus groups. Looks like they’re doubling the sperm count and cutting the recovery time. And introducing multitasking.
And from what I remember reading, the “afterglow” of the orgasm actually induces the female to remain prone for an extended period of time, thereby ensuring that more of the happy juice gets taken in and not spilled upon standing.
That’s a 3D movie I wouldn’t like to watch “Hunter-killer sperm on the attack - (You’d wish you’d never have come)”