What might cause the split between people who think toes are ugly and toes are pretty?
People have different experiences that lead to different behaviors. But it's not like my past experience in thinking some women are ugly causes me to think all women are ugly. Why should that be the case for any aesthetic experience?
Thankfully the questioner gave me an out by asking what might cause some people to find toes pretty while others think they're ugly.
I'm going to use that to my advantage to give a completely unsubstantiated, just-so answer that's too cool for me to ignore. If more scientific evidence comes out on this topic I'll try to remember to adjust my answer accordingly.
Short answer: toes and penii might be closely related (neurologically speaking). For more science (SCIENCE!) read on.
In my answer to Why can't I control my individual toes? (two toe-neuroscience answers?! o_O I'm not even into toes!) I introduced the motor homunculus:
This guy's body parts are distorted such that the size of a body part is proportional to the area in the primary motor cortex that is dedicated to representing that part. This was first determined by Wilder Penfield by stimulating people's brains and mapping the motor responses of the body.
Just behind the primary motor cortex (blue in the figure below) is the primary somatosensory cortex (red). The somatosensory cortex is the final common pathway for all incoming touch sensations of the body (pain, light touch, etc.)
The representation here is similar the motor cortex and mirrors it quite closely. Save one pretty striking exception.
Meet the (male) somatosensory homunculus:
If you're up for it, here's the uncensored, possibly NOT WORK SAFE version (if your workplace hates science).
The first thing you'll notice is that the representation of our toes is much bigger on the somatosensory compared to the motor homunculus. (That was the first thing you noticed, right?) This means our toes are given a lot of brain area in the somatosensory cortex, which means we have relatively more sensitivity in our toes than, say, an equivalent area on our shins.
So lets take a look at the somatosensory map to see what the layout of body parts looks like on the brain:
Check out the locations of the toes there on the right. At the top, where the butt is, is the top of the brain. This image represents only one half of the brain, so the butt is actually at the top center, and right across from it, in the other half of the brain, the other half of your butt is represented.
This means that the toes are actually represented on the medial surface, squished between the two hemispheres of the brain (with each hemisphere having a representation of the toes on the opposite side of the body).
See what body part is represented right next to the toes, though!?
GENITALIA! Yay! I'm so close to actually answering the question now!
(See Why does the writing style of most PhDs on Quora appear to be long-winded and poorly structured?, bro.)
Here's the theory put forth by UCSD neuroscience rockstar VS Ramachandran:
In some people, neurons coding for or representing genetical sensation are "cross-wired" with neurons representing toes and feet. This cross-talk may give rise to the sexual associations of toes and feet.
This is far from proved, but it makes for a nice story. Ramachandran has done some clever experiments to test his theories about how neuronal "cross-wiring" gives rise to certain behavioral phenomena, such as synesthesia, so it's not a totally out-there hypothesis.
Ramachandran relates an amusing story of one of his patients about this topic:
The next day the phone rang again. This time it was an engineer from Arkansas.
"Is this Dr. Ramachandran?"
"You know, I read about your work in the newspaper, and it's really exciting. I lost my leg below the knee about two months ago but there's still something I don't understand. I'd like your advice."
"Well, I feel a little embarrassed to tell you this."
"Doctor, every time I have sexual intercourse, I experience sensations in my phantom foot. How do you explain that? My doctor said it doesn't make sense."
"Look," I said. "One possibility is that the genitals are right next to the foot in the body's brain maps. Don't worry about it."
He laughed nervously. "All that's fine, doctor. But you still don't understand. You see, I actually experience my orgasm in my foot. And therefore it's much bigger than it used to be because it's no longer confined to my genitals."
It may be that neuroplasticity after this patient's limb loss induced communication between his foot and genital sensory neurons. This observation lends some support to Ramachandran's toe/brain/penis hypothesis.
For more on cool stuff related to neuroplasticity, see my answers to Can brain trauma cause cognitive enhancement? and When parts of the brain are removed during surgery, is it possible for the remaining brain tissue to expand into the available space?
For more reading on sex, brains, and homunculi, check out the neurocritic who's covered recent neuroscience research looking at the somatosensory representation of circumcised v uncircumcised male penises, the representation of the female homunculus, and the neuroscientific attempt to find the clitoris.
Hubbard EM, Brang D, & Ramachandran VS (2011). The cross-activation theory at 10. Journal of Neuropsychology, 5 (2), 152-77 PMID: 21923784