This is going to be a heavily chemistry-based rant, just for the record. So, I've been thinking a lot about the neurochemistry behind euphoria and sexual stimulation. It's difficult to get a full picture of what's going on in the brain because we only know so much about it, but here's what I'm seeing so far.
Dopamine is one of the main chemicals involved in how horny you are and, by extension, how willing you are to do crazy sex stuff. Dopamine in the nucleus accumbens and other parts of the mesolimbic pathway lower the activity of the prefrontal cortex, which puts you more in an instinctive goal-seeking mode and open to suggestions. This is how dreams work, with dopamine levels in this area being at their highest at that time. That's also why things like sex and flying are the first things that come to mind in lucids. Serotonin is one of the main chemicals involved in feelings of happiness, acceptance, and other generally positive emotional states. Dopamine and serotonin both work together and against each other in different parts of the brain and are both very big players in consciousness as a whole. Norepinephrine also plays a role in each of these functions to an extent, but the most significant aspect of what it does here is creating the physical aspects of sexual stimulation - increased heart rate and perspiration, the butterflies in the stomach feeling, and etc. Then, you've got the main sex hormones, testosterone, estradiol (estrogen), and progesterone. I'm not sure how much of an effect progesterone has in all this, but the other two are definitely big players. Testosterone increases natural dopamine levels and estradiol increases natural serotonin levels. The end result of this of course is that men are pretty often ready to go but women are more likely to need to be worked into it through means of increasing dopamine somehow.
Another big sex hormone is oxytocin, and it's significant in both sexes but has some differing effects in each. Oxytocin is actually a chemical with a wide range of effects and many pro-social outcomes both positive and negative, but in this situation the easiest way to describe it would be to say that it's the bonding hormone. It's the thing that makes you want to stay with someone you love even after the initial rush of being with them is over. Its effects are also much more significant in women, as it's enhanced by estradiol and inhibited by testosterone. The most straightforward way to increase oxytocin is physical contact, like kissing, fondling, groping, and all that good stuff, but also through things like hugging and holding hands. It gets released in other ways too, but that's the major aspect here. Oxytocin also increases sex drive, which of course makes sense. And that alone is enough to say on the surface why stuff like foreplay is useful for women, but it doesn't actually get into the deeper aspects of it, like why it's that way on a more basic level.
Lately I've been reading more into the interactions of the endorphin, endocannabinoid, and GABA systems. These systems constitute the big three classes of "downer" drugs, the ones that can turn your focus inward rather than pushing it outward. I'd been looking specifically into how CB1 receptors increase the release of enkephalins, the endogenous ligands for the delta-opioid receptor, and how this process may be involved in a feedback loop that causes alcohol consumption to increase the more you have to drink at once (basically, why being really drunk just makes you want to get even more drunk). In my research I found that activation of delta-opioid receptors actually lowers levels of oxytocin, and since all the big drugs in the three downer classes activate those receptors either directly or indirectly it makes perfect sense as to why they can make you less social as the doses increase. But what's interesting about what that led me to is that it would seem that the mu-opioid receptor is actually involved in the euphoria caused by oxytocin, and enkephalins themselves activate mu receptors, if to a lesser extent than delta receptors. The implication here would be that oxytocin and enkephalins are meant to cause the same type of euphoria while having opposite effects on social behavior, which is not too crazy. I've seen similar patterns in the relationships between other systems before.
In addition to releasing endogenous opioids, CB1 receptors also increase the release of phenethylamine. This is something they seem to share with mu-opioid receptors, and in fact there are areas in the nucleus accumbens where the two receptors bond together to form couplings. I can't say for sure, but I'm guessing that this phenethylamine release is the main thing that they share together, as their other effects seem to be mostly related to balancing out the opposite system. Phenethylamine is another big sex chemical, and supposedly plays a role in immediate sexual attraction and the first few years of a relationship, before oxytocin starts becoming the most significant thing. The simplest way to explaining it would first be to point out that cocaine and amphetamine are two related but distinct types of stimulants and amphetamine is the significantly stronger one, and phenethylamine basically effects the parts of the brain that amphetamine does but only at the level of how cocaine does it. It's our brain's own personal stimulant, and it increases levels of dopamine and norepinephrine, and to a lesser extent serotonin. What's interesting is that both stimulants in general and opioids have been found to have their rewarding dopamine releases modulated by 5-HT2A and 5-HT2C, which are serotonin receptors. 5-HT2A induces the release of dopamine in the nucleus accumbens and 5-HT2C inhibits it, and stimulants alter the activity of each by directly preventing the dopamine from being removed from the synapses. This basically means that the rewarding dopamine release from serotonin shifts in a positive direction. Since phenethylamine works just like a stimulant, it makes perfect sense that those rules would apply to it, and could possibly even explain why mu-opioid receptors were found to follow the same path if one of their main effects is to release phenethylamine.
So now let's tie it all together. Men naturally have high dopamine and women naturally have low (relative) dopamine, which is why men are naturally much hornier than women. Since most women need some form of stimulation first, things like foreplay will release oxytocin in the brain which binds to oxytocin receptors and then works downstream through mu-opioid receptors. Mu receptors increase the release of phenethylamine in the nucleus accumbens, which inhibits the reuptake of dopamine and increases the rewarding properties of 5-HT2A receptors. Meanwhile, things like feeling loved or accepted increase the woman's mood and that releases more serotonin, which reaches the nucleus accumbens as well. That serotonin then binds to both 5-HT2A and 5-HT2C, and since the ratio of dopamine release has shifted those emotional states then elicit much more powerful euphoric responses, and that causes sex drive to increase. And that's why all that physical and emotional intimacy can make such an important difference.
And now, finally... my rant is that I put all this time into learning these things and trying to work things about and yet I still can't prove anything. >:T It all makes perfect sense to me, but as I've ranted about before, just because something seems to fit perfectly doesn't mean it's actually correct. Why can't we know more about these things? 
But on the other hand, if it's true then it's cool to know. I've been trying to tie all of the different classes of euphoric drugs together in the brain for a long time, and if I'm right then I think I'm getting pretty close to that goal.
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