So, any chance they would reject the tag “accidental penis shaming”?

And then Voldemort begain a crusade against the ‘Mud(blood)pony’s’ stallions

@Darkfur  
You’re comparing weight to length. That’s a problem, because when you scale up the dimensions of an object, you’re scaling up length and width and height. If you doubled the length, height, and width of a 180lb man (and kept density the same) you wouldn’t get a 360lb man. You’d get a 1440lb man. Twice as tall, twice as wide, twice as thick –> eight times the volume –> eight times the mass. A man who was seven times the normal size in all three dimensions wouldn’t be the size of a horse – he’d be the size of a house.
 
As you say, horse are about 5 to 12 times larger (in weight/mass) than the average man. To match that weight, a man would have to be ~1.7 to 2.3 times bigger in each dimension. That means about twice as tall, twice as thick, and twice as wide.
 
I’m told by size charts that average full-size horse endowment is modestly more than twice as the radius of the average human male endowment, and about four times as long. A stallion scaled down to the same weight as a human would average half those dimensions, meaning a modestly thick dick with astonishing length.
 
I personally like that, because (if my little stallions are about half the dimensions of real-world horses, which matches the candy-cane metric) that makes FiM stallions well-endowed without being ridiculous.
 
(Of course, porn pictures usually go for dicks that are maybe 50% larger than average. A my little pony with a big dick for a my little pony would be a tad ridiculous. Like, ‘extra large Chance’ ridiculous.)
 
This is assuming My Little Ponies have the same proportions as horses, which of course they do not.

@Background Pony #915D
 
To complement what @Darkfur said, your point 3 is a fallacy. If the human size, and therefore surface area to stimulate a number of Celestia’s neural terminations responsible for pleasure, is lacking, there is only one thing left to make the pleasure build up and that is the length of the act.
 
Therefore, it’s only logical that a human can last longer than a stallion, long enough to make a mare like Celestia reach her climax.

@Background Pony #915D  

(1) Real-world horse dongs are quite a bit bigger than human dongs even relative to body size. Scale a horse down to a human’s body weight and you’re still packing a big dong. Thicker and much longer.
 
(2) Equestria’s ponies are canonically much much much more flexible than real-world horses. They’re clearly flexible enough to go balls-deep. (Of course, one could argue that since they can go balls-deep, they wouldn’t need or have that extra equine length.)
 
(3) The sexual endurance of feral real-world equines has no bearing on the sexual endurance of sapient civilized magic ponies. Yes, horses have crap sexual endurance. So do bonobos. Bonobos are much closer to humans than horses are to FiM ponies. So, by the same logic, humans must have crap endurance. Durr.

Poor Lord Voldemort.

@Jack Kellar  
Thank you for this knowledge. Tha more you know.

@Background Pony #2669  
I’m not entirely sure the first point applies to Equestrians, but let’s say it does.
 
We can say that, since the sie doesn’t matter but hilting ability does, a human male’s endurance would absolutely floor any mares and humiliate the stallions. Mares are quick to reach orgasms too, so we’d give them multiple ones.

@ALIS  
@Background Pony #2669  
Both of these are the only argument you need.

@KR4K40  
Because the guy from the comic I based this on is wearing underwear in the last panel.

…why is he wearing underwear?

@Background Pony #2669  

Don’t forget two important horse facts:
 

@ALIS  
No wonder all those mares fall in love with all those humans in fanfic,and I thought it was all just stupid self inserts,silly me.

By size comparison to Celestia he should be as big or bigger then a normal sized pony stallion.
 
Horses are not big proportionally to humans, they are big because they are 5 to 6 times heavier. Celestia in this is smaller then the average horse about the same mass as the human. She is again twice as big as a pony stallion.
 
So no he should be bigger then a stallion.

the first time? wow, that´s one miserable life

It’s not a matter of size; it’s how you use it. A horsecock isn’t worth jack if fucking with it lasts all of ten seconds.
 
Humans must be gods of sex to ponies, now that I think of it.

Celestia: “If it’s any consolidation, Humans have the largest phallus of any primate.”

Yeah, seems about right.