There is definitely a physical desire and reaction and anticipation, but I agree that the psychological is stronger, for me anyway. This is a very interesting thread and I have been thinking alot about it.
A key moment in my sexual awaking was a trip to the pediatrician's office for my annual checkup before school started, probably 6th grade or so. The psychological aspect of it haunts and stimulated me to this day. My doctor was a middle-aged man, not that I had any attraction to him, I'm heterosexual. As the exam progressed and my clothes came off, I began to get feelings of anxiety, embarrassment, and something probably related to being turned on sexually in a way, feelings I have no memory of in any previous doctor visit. The moments which are most clear begin with me being on my back on the exam table, wearing only my underwear, as he palpated my lower abdomen, which I felt was already getting a little too close to the private parts of my body. Then without saying anything his fingers curled under my waistband and pulled my underwear down just to my thighs and began to examine my penis and testicles. This caused my whole body to feel very hot all of a sudden and I remember being shocked at the sensation of his hands and fingers on my private parts. I didn't get an erection although I was completely stimulated. My psyche was electrified like never before. I remember just staring at his face as he looked at my genitals and examined them. Then he stopped and pulled my underwear back up and asked me to turn over onto my stomach.
As I lied down, my brain was spinning. My feelings of being out of control, embarrassment, excitement were intense. My 12 year old mind started to wonder and make the connection that if the doctor just examined my very private frontside so intimately then perhaps the same was about to happen to my backside? Or maybe not I thought since he was only examining my spine for a while. And anyway how did I feel or think about what might be about to occur?
Suddenly I felt his fingertips again slip under my waistband and I sensed and hoped even, what was about to happen. Though it was years ago I can still remember the surge of something, maybe adrenaline, shooting through me. It felt like my blood got instantly hot and my brain was out of control on alert. And he lowered my underwear again to my thighs, baring my bottom. I could feel the cold air of the doctor's office on my skin and I remember thinking for the first time consciously someone is looking at my naked bottom and being very, very excited about it. So the physical was there and was a big part of my reaction, but it was my mind that was the most stimulated by far. But at the same time, I was apprehensive, a bit scared. The next thing I felt were his hands on my cheeks and quickly he spread me wide open as he examined inside my bottom. I'm not even sure at the time if I knew the word ANUS but I could feel that same cool air touching that normally unexposed area as I lay there completely open to the doctor's eyes. It was a feeling unlike anything I had felt up to that point in my life, a feeling emotional, psychological, physical, all things I realize now, but then it was just a confusing, intense, hot, sort of shameful, and incredible feeling all at once. I'm sure this part of the exam didn't last more than a few seconds although it felt much longer. It wasn't a rectal exam. He just looked. Which was almost more than I could handle psychologically then anyway. Quickly my underwear was pulled back up to my waist and he continued with the rest of the exam, not that I can remember anything else that afternoon except for those amazingly unexpected and intimate moments which I thought about for the rest of the day and for the rest of my life obviously.
From that exam there was no physical orgasm, not even an erection, but in a way, my brain orgasmed and my psyche was changed from that day. I wanted to know what the doctor saw, what he was looking at. I wanted to relive that experience somehow. That night was the first time I recall looking at my own anus. I had my own bathroom with a wall mirror, so I bent over and spread my cheeks and took a look. I'm not sure what I expected to see. It was really very simple, pale and pinkish skin in between my cheeks and a small pink wrinkled spot in that crevice (at the time I was completely hairless). It didn't even look like a hole really just a pink opening. Over the weeks and months I would sometimes go into my bathroom and examine myself, when no one else was at home. Eventually, I started touching myself there, then touching my penis as it became hard. I would spread myself wider, pulling with my fingertips the hole itself and noticed how very dark pink my anus is inside - the rest of my body is very pale. Over time I began to push my finger inside just a little bit, to see how it felt, something the doctor had not done. Sometimes it felt good, sometimes not so much. Regardless, it was intensely exciting to "examine" myself in that way, to imagine someone else doing it, to know that it might happen next time at the doctors office. I began fantasizing too. Sometimes I would imagine I was being watched by one of my cute classmates from school. I would get excited imagining that one of those cute girls might be doing it to me or maybe the same thing to herself at home in the mirror too. I wondered if that was possible or was I the only person experiencing these thoughts and desires. After these sessions were over I felt guilt and shame. I always thought I was weird and bad for these thoughts and actions. But they continued of course. I only started to feel psychologically positive about my desires years later when I met and dated women who had similar desires, though not all did for sure, and one girlfriend in fact was severely almost violently alarmed by this aspect of my sexuality. On the other hand, there were a few girlfriends whose cravings matched mine, who understood the desire psychologically like I felt it, who I could talk about it with openly and without shame.
So yes, the psychological and emotional aspects always have been present from the very beginning and are still very strong today.