Sorry for not responding sooner. For the neurological reasons, it could fill several books and could have a higher-level university course (i.e. third or fourth year) devoted entirely to neurological reasons for various sexual orientations. So, I'm not going to write an incredibly complex post, mostly because I may not understand what I'm writing and that's useless to you, others and myself, I'm only going to write what I do understand. Since I understand a decent amount and have done some research papers, including one of functional neuroanatomy (although not for sexual orientations), I'm going to write this post with the assumption that you have little knowledge of human anatomy and physiology. This isn't meant to be taken personally, it's simply meant so I have a base to start my explanations at and to ensure that the base isn't above your head; if it's at a level you already understand, then perfect.

When sensory information (i.e. sight, sound, smell, etc...) is taken into your nervous system, it goes along the appropriate pathways depending on the type of sensory information but eventually it makes its way to a structure in the brain called the thalamus. The exception is smell and although smell can be part of the explanation, I'm basing my explanation on the premise that the sexually-appealing material is based from online resources, that is, you're not engaging with someone who appeals to your desires. It may seem slightly obscure as to why smell is involved, however, the basic explanation for smell is that as living organisms, we produce various biochemicals that can be detected by other organisms of the same or different species.

When you view sexual material that's arousing to you, as I mentioned above, it reaches the thalamus. The thalamus can be divided up into various zones but I'm going to ignore that for the purpose of this post and orient this post to less in-depth explanations (unless further in-depth ones are required).

After it has reached the thalamus, the information travels to various parts of the brain. One part that it reaches is called the nucleus accumbens or NAcc. This part of the brain is part of the pathway called the mesolimbic dopamine pathway. This name sounds like a mouthful so I'll break it down. The limbic portion refers to what's called the limbic system. Unlike most neuroanatomy, there isn't one defining structure called limbic or limbic system; it's composed of various parts, such as the NAcc, thalamus and other parts.

The NAcc is involved with reward, pleasure and is involved with conditioning, more specifically, with Pavlovian or classical conditioning (named after its founder, Ivan Pavlov). It's also involved with explaining addictions because of the immense pleasure and conditioning it's involved with.

The rest of the pathway mentioned above has to do with dopamine or DA, a very common and dispersed neurochemical. Here is where things can get very complicated and messy because in the brain, there are numerous pathways and unfortunately for scientists and us, these pathways are interconnected and interrelated in very complex ways. If we were to try to devote one colour per pathway in hopes of trying to sketch out the actual pathways in 2-D or 3-D, we'd end up with a multi-coloured circle or sphere. Part of the reason for this is that DA is produced through a series of enzymes and other chemicals, and each of those chemicals have their own unique pathway, and each enzyme has its own unique pathway.

Focusing on DA though, it's involved in pleasure, happiness, reward and many other things, however, I'm only going to focus on these types of reactions. As I mentioned above with the limbic system, another part of it is the amygdala. If you've taken Latin or Greek, you may know this as the word for almond because it looks like a cute little almond (yes I said part of the brain looks cute, LOL). This part of the brain is involved with regulating emotions and this includes happiness, pleasure, etc... .

Recall that the brain has numerous pathways and because DA is a common one, it is involved with another major one, the serotonin or 5-HT pathway (5-HT is an abbreviation for its full name, which is 5-hydroxytryptamine). 5-HT, like DA, is involved in pleasure, happiness, reward, conditioning, etc... .

By now you may have realized that there's a common theme to the functional neuroanatomy: pleasure, reward, happiness and to an extent, classical conditioning. The reason is, that if you fancied homosexuality, heterosexuality, sadism, etc..., these same parts, along with many others would be activated and when they're activated, they produce the sensations of pleasure, happiness, etc... . However, do remember that the NAcc is involved with addictions and conditioning, and it is one the big players in the neurological explanations. If you have, say, the amygdala activated, then you may feel happy or sad or whatnot, but if you have the NAcc activated, you've kicked it into "high gear" and now you're really feeling pleasure, reward and you're wanting to view whatever the stimulus was again because you want those feelings over and over again.

Another part of the neurological explanation is an incredibly fascinating phenomenon of neuroplasticity, that is, parts of the brain physically changing regardless of your age. When you have the NAcc activated over and over again, remember that it's not simply that one part being activated; there are pathways being activated (i.e. mesolimbic dopamine system), neurochemicals being synthesized and released, etc... . As this continues over and over for certain areas, those areas can develop more connections and so when those same stimuli come again, the reactions you feel are intensified.

One other pathway that I'll mention, although it involves a lot more anatomy and physiology is the hypothalamic-pituitary-adrenal axis or HPA axis. This part involves a part of the body that you may not suspect at first: the kidneys and believe it or not, the HPA axis is involved with pleasure, emotions (including depression), sexual responses, stress, etc... . So put lightly, your kidneys are involved with depression and other moods.

The HPA axis involves the pituitary gland, which is the "master gland" because it releases so many chemicals. In fact, it's been divided up into posterior and anterior parts and each part releases numerous chemicals, specifically, it releases many hormones, more so than most other parts of the brain. Another part of the name of HPA axis is the hypothalamus, which has so many functions, it's easier to just say that it's involved in regulating bodily functions. If I were to list the functions it does, I'd be listing a large paragraph, it's that amazing! For this reason, when the hypothalamus is activated as a result of the sexual stimuli, numerous other parts of the brain and body are activated. For example, when you view the sexual stimuli, do you perhaps get sweaty hands, do you feel your body temperature get warmer, does your heart beat faster, etc...? All of those and more are attributable to the hypothalamus.

The heart beating faster in particular can be due to one other part of the body: the vagus nerve. There are 12 cranial nerves (large, major nerves stemming from various parts of the hindbrain to all over the body). The vagus nerve innervates not only the heart but also the lungs, and so when it is stimulated, the heart rate increases, you breath more, etc... .

The name of the HPA axis includes one other part, the adrenal gland. Before I mention what it does, there's a chemical you probably know of already that sounds familiar. That chemical is adrenaline and when it is released, well, I'm sure you know what it feels like.

So, how does all of this relate to the sexual stimuli? Well, when the information comes in, it reaches the thalamus, which sends information to all over the brain. The hypothalamus sits very close to the thalamus (note the prefix "hypo-") and that gets activated, however, because it tends to involve other organ systems, it takes a bit longer. The time difference is one you probably cannot feel because it's a matter of milliseconds so overall, it may seem like it occurs at the same time. The thalamus is connected to the limbic system and the amygdala and NAcc get activated, leading to the experiences of pleasure. The hypothalamus along with the rest of the HPA axis releases hormones and activate various parts of the body, such as lungs and heart via the vagus nerve or cranial nerve ten, or N X.

But, there's one major part I've left out so far: the hippocampus. This part is mostly concerned with memory and the reason for involving the hippocampus is simple. When the stimuli of any sort come into the brain, you have the think, how to react, and this is where the hippocampus comes into play. The reason I left it out until now is that it's probably not something that requires immense explanation because when we deal with memory, we move from biological/physiological psychology (i.e. neurology) and move to cognitive psychology, which although it does involve neurology, it also deals with more abstract concepts. Cognitive psychology has countless books and there are so many types of memory that I think it'd make this post seem even longer and more complex than it already is.