I do a lot of research on this topic so expect this to meander:
Reay Tannahill, a Scottish Historian, makes some interesting observation on beauty, and the mating posture.
Many female animals present their rear, the male mounts and it's over quickly. It is an instinctual and purposeful action. She suggests that the very act of humans biology making it comfortable to mate face to face had a strong impact on human sexuality. Eyes connect, the vulnerable face is open to your partner, allowing our tongues to touch while united. Nerve endings, sensitive tissues and our senses engage in a very different experience, one that can allow for emotions of intimacy to develop.
In "Partners of Sexual Behaviour" anthropologist and ethologist suggest that the very angle of this position made female orgasm possible - a treat believed to be deprived of our fellow critters.
This frontal posture also made the female human more susceptible to rape, something physiologically impossible for most other species. Darwin's theory of evolution is often thought of as being strictly based on natural selection. However he also acknowledged sexual selection. Those attractive to females would mate the soonest, and the most often. However, since the female human can be raped - our genetic and social history is forever changed. War opened up large masses of women to rape, and so did a male dominated society - the female did not need to approve of her mate in order to be impregnated.
In fact over time evolution compensated for this act of violence against women. Studies show that a majority of women watching rape scenes with sensors attached began to lubricate, even though they found the scenes appalling and experienced no sense of arousal. The belief being that women who could lubricated without being turned on, and even in the midst of violence, were less likely to die from certain internal injuries, and were less likely to have injuries that would render them infertile - therefore passing on the ability to more and more women.
This ability to rape women is likely responsible for the highly aggressive behaviour of human beings across the planet, as we continue to kill each other for the entire span of recorded history - behaviour unknown in the animal kingdom. Males who manifested the traits that would lead to the destruction of neighbouring peoples and raping their women were able to pass on their genetics. Sex and Reproduction being linked deeply to War and Violence. Granted it doesn'tmean we're inherently murderous or that war is inevitable. Part of being sentient means that we are not ruled by blindly by our animal instincts.
Interesting isn't it? The face to face position opening us up to great intimacy at the same time it opens the door to great violence.
Tannahill also points out that early hominids likely had sexual selection effected being mounted from the rear. The woman had no real view of of the man for most of the process - making his actions more important - strength and intelligence being favored. Whereas the male had full view, making it more important that she be visually appealing.
There is an interesting (and small) body of research that tries to see if animal females are able to experience orgasm. There is evidence that many female species experience strong contractions during mating, but there is no way to tell if she's experiencing physical ecstasy. Human semen contains prostaglandin, a hormone that causes a woman's uterus to contract when it comes into contact, and it's possible something similar is true for animals.
So... in 1939 Illinois researches took to sexually stimulating rabbits to take semen out of the equation, apparently using a finger. What fun. A dye was injected into the rabbit's vagina. They learned with the use of dye that these horny little bunnies did indeed experience contractions. Then in 1952 another Illinois research group inserted the thumbs of latex gloves into the uteri of cows and filled them with water... Interesting people in that state. Uterine contractions were detected the moment a bull was brought into sight. Another curious observation, the same study that showed women could lubricate to scenes of rape even though unaroused by the footage, would also lubricate when scenes of animal sex were displayed though there were no other indications of arousal. Merely being in the presence of sex of any kind triggered the female body to prepare itself.
Another study was done by a graduate student named Carly Kendall, for his master's thesis "Orgasm in Female Primates" who conducted the research by:
"manually stimulating the circumclitoral area and vagina of several adult, adolescent and juvenile chimpanzee females.... During intravaginal stimulation, perviaginal muscular contraction of about .8 second duration and about one second apart were palpated The average number of digital thrusts (at an approximate rate of one to two per second) performed before the onset of the contractions was 20.3. On one occasion... this female reached back to grasp the trusting hand of the experimenter and tried to force it more deeply into her vagina."
Odd to think of all these professionals diddling animals. So it took the female about 15 seconds to 'orgasm' on average, but unfortunately the male chimp takes 5-7 seconds to finish the job. Meaning that even if the female chimp can orgasm, she probably never gets the chance to.
Problem is, that for all these contrations being measured, there was no change of expression or body language in the female animal to suggest bliss. So of course endocrinologist D.A Goldfoot finds that the stump-tailed macaques make a "round mouthed ejaculation face", but primarily when females are mounting other females with thrusting motions. So naturally they inserted some instrument into a female uterus, and it turned out there was a link between the facial expression and what was going on in the body.
And then of course we have the ever famous dolphin:
Dolphins are known to have sex for reasons other than reproduction, sometimes also engaging in homosexual behaviour.
Sex is such an interesting puzzle, so much of life (animal and our own) revolves around it.