Why is "ass" considered obscene?

Why is ass considered obscene?

When ass is used to refer to the animal, it's generally not considered obscene. Some would consider it obscene when referring to the human posterior, or more likely, when used as an epithet for a boorish or stupid person. Asshole in my view is considered even more obscene.

The "why?" question is pretty subjective. Why is any word considered obscene? It's just the way the word developed and societal convention.

I believe in the spectrum of obscenities, even as an epithet it's not that obscene. In my opinion, a filter which replaces the word with butt is too stringent.

What is the difference between ass and butt?

When referring to anatomy, there's little difference. Each word has other senses, however. An ass can refer to the sure-footed animal smaller than a horse with big ears. It can also mean a pompous fool or be used as slang for sexual intercourse (get some ass).

Butt has many other senses. It can be the part of the trunk of a tree nearest the roots, the victim of ridicule (butt of your jokes), thick end of a handle or stock of a rifle/pistol, the end of a cigarette, or a type of joint (carpentry, not the other type of cigarette.)

Butt can also be used as a verb, to place end to end, to lie adjacent, or to strike or shove against/in between, (to butt in line.)

I will answer to the second question.

The words have different meanings; leaving out what the words mean when they are used as verbs, these are the definitions of the words:

butt /bət/
noun
the person or thing at which criticism or humor, typically unkind, is directed: his singing is the butt of dozens of jokes.
• (usually butts) an archery or shooting target or range.
• a mound on or in front of which a target is set up for archery or shooting.

noun
a push or blow, typically given with the head: he would follow up with a butt from his head.

noun
1. (also butt end) the thicker end, especially of a tool or a weapon: a rifle butt.
   • the square end of a plank or plate meeting the end or side of another, as in the side of a ship.
   • the thicker or hinder end of a hide used for leather.
2. (also butt end) the stub of a cigar or a cigarette: the ashtray was crammed with cigarette butts.
3. informal the buttocks.
   • the anus.
4. the trunk of a tree, especially the part just above the ground.

ass /æs/
noun
1. a hoofed mammal of the horse family with a braying call, typically smaller than a horse and with longer ears.
    • Genus Equus, family Equidae: E. africanus of Africa, which is the ancestor of the domestic ass or donkey, and E. hemionus of Asia.
   • (in general use) a donkey.
2. (informal) a foolish or stupid person: that ass of a young man.

I'm sure you know the different meanings of ass. For the vulgar meaning it comes from the reconstructed Proto-Indo-European "Orse" which means (“backside, buttocks, butt”). In this meaning it's widely used in North America.

Note: Also it has some other word origins.