It's also important to understand the underlying evolutionary purpose of orgasm. For males, orgasm is both the mechanism that transfers sperm into a female and the psychophysiological reward for managing to do so. Male sexual arousal begins with the sight of a nubile female, and it proceeds directly from there to orgasm. The human male urge to thrust comes because the human penis shaped to pump sperm out of the vagina, in order to clear out the sperm any competing male before the thruster rejects his own. For this reason, most males have single orgasms. There's no reproductive benefit to pumping out all the sperm you just ejaculated. Not only that, but because sperm is a limited resource in the short-term, there is evidence that men finely adjust the volume of their ejaculate to the circumstances of their relationship with the woman they are inseminating. In an ongoing relationship sperm is metered carefully, but in a new encounter, or after a period of separation, the amount of sperm and ejaculate goes up steeply. In evolutionary terms, this is because in these encounters there is a higher likelihood that the woman has been inseminated by another man. Of course, nobody does this consciously, but every man knows the excitement of a new conquest, and the passionate sex of lovers reunited after an absence. This leads both to harder thrusting and stronger orgasms, which minimizes the presence of a potential rival's sperm, and maximizes the presence of his own. If the sperm of two different men come into contact the battle continues inside the woman's body, with specialized hunter-killer and blocker sperm seeking to take out the rival's swimmers before they can reach the egg.
For a woman, orgasm is different, and everyone knows that while the typical man enjoys a straight line track from arousal to ejaculation, a woman's sexual response can vary from no orgasm at all to dozens of orgasms in rapid succession. Again, the key is in the evolutionary purpose of orgasm. On the most basic level, at the moment of orgasm, the uterus contracts repeatedly, and the cervix pulses up and down at the top of the vagina. In effect, it acts like a turkey baster, sucking up the sperm to help it on its way to the egg. This is straightforward in most mammalian species, and all the average mammalian female has to do is select the most genetically superior male to get her pregnant. However, humans are more complex. Because human children need so much care for so long, it is nearly impossible for a woman to raise one alone in a primitive setting (which is where all these instincts evolved). She needs help, and this can only come from those who share a genetic interest in her child – her parents, siblings, and family, but most of all the child's father. This is unique to humans. Paternal child-care is unheard-of anywhere else in the mammalian world. In every other species, the males compete to be the alpha, and the winners then proceed to find – fuck – forget all the females they can lay hands on. And this unique feature of our species is where it gets interesting.
Ideally, a woman wants to be impregnated by an alpha male and have him help raise her children, but by definition there are far more women than there are alpha males. Not only that, but because alpha males are in such demand, they have options. Sperm may be a limited resource in the short-term, but in the long term a man can get dozens, hundreds, even thousands of women pregnant. If a man can pull this off, it is a very successful genetic strategy. He's got no reason to tie himself down to any one particular woman. Because of this, most women will then have to settle for a beta male to father her children – unless she cheats.
A woman cheats by forming a pair bond with a beta male, and then going off to fuck alpha males behind his back. This way she gets the best of both worlds, the best possible genes for her children, but the ongoing help she needs to raise them successfully. To make this work she has to convince the beta male that he's at least a chance of being a father – which is to say she has to have sex with him. That means she has to enjoy it at least somewhat, at least long enough to get him committed. It is a rare woman who does not have tender feelings for her beta male. However her strongest sexual response is reserved for her encounters with an alpha male. In fact, it has been shown that a woman's sex drive varies with her menstrual cycle. She prefers gentle, tender (beta) lovemaking when she is not fertile, and is attracted to physically beta types during this time, but when she is ovulating she is attracted to larger, stronger, more masculine men, and she wants rough, dominating sex from them (which verifies to her subconscious that he is indeed an alpha). And more than anything she wants those hard, pounding, deep thrusts that pump her beta lover's ejaculate out of her, and prime her for the soul wrenching, cervical orgasms that will suck all the alpha male's sperm directly up to her egg.
And here we have the complete picture of the promiscuous female, her helpless cuckold and the bull who owns her pussy. Initially she has sex with the cuckold, to cement the pair bond. Once that is accomplished, she grows restless, reduces sex with him and begins to actively hunt for the alpha bull her ovaries are demanding. She finds him, and the sex is explosive. The reason it's so good is because it is perfectly meeting both of their needs – his to enjoy limitless pussy with no strings attached, and hers to be alpha-fucked while keeping her beta-provider on a short string. And what of the poor cuckold? He's making the best of his genetic situation too. He's a beta male, so he can't easily replace his woman. He can't even hope to keep his woman from straying, though perhaps most men are in denial over this. All he can do, if he gets past the denial, is accept what she allows him, in hopes of keeping her around and possibly fathering one of her children by accident. And here, too, his instincts are guiding him. His most intense sexual response comes after she gets home, her pussy full of the alpha's sperm. Its separation sex to the nth degree, and so he fucks her as well as he can, orgasms as hard as possible. Of course he likely has a smaller cock and less sperm in his ejaculate, and he certainly can't trigger the deep orgasmic response that the alpha gives her, so he's fighting an uphill battle.
Of course this is painted in broad brush strokes. Sociopsychosexual behavior is complicated. There are endless variations on the theme, and many questions that research has yet to answer.