It's common decency!

Sometimes you encounter these cases where someone asserts a position, and when questioned, states that it is the right thing to do "because it's common decency".

It is common decency to do/believe in/accept X

Therefore X is good/true

Example:

"Why should I respect your self-image at all?"

"It's just common decency, it costs you nothing to be nice!"

You can find this kind of reasoning all over, but what mistake is being committed here? Surely everyone has a different idea of what common decency actually is? It seems a bit similar to the appeal to common sense. 

Dr Bo is correct. 

The argument in its current form is not rationally persuasive because it implies that something being "common decency" is "good", when neither of these parameters are examined.

It may also count as a thought-terminating cliché as the aim is to stop critical examination in favour of "common decency". Sometimes, over-tactfulness and fear of offending can cause damaging falsehoods to be propagated, and, although uncomfortable, it is necessary to bring truth to the table.

This seems to me like a kind of appeal to normality , because common decency is certainly a social norm.

The fallacious Appeal to Common Decency (Sense) is one thing, but there are more complicated fallacies in the overall OP that should be examined.

"Why should I respect your self-image at all?" is actually NOT an example of 

' It is common decency to do/believe in/accept X"  as it diverts attention from "do/believing/accepting" a particular belief (X) to the opponent's disregard for their self-image. 

This is a form of a red herring, non-sequitur, and possibly a loaded or complex question in that the overall structure of the question assumes that belief and self-image are somehow related.

Doing/believing in/accepting a belief has nothing to do with another's disregard of their self-image. They could quite simply be arguing: y ou are not what you think. Which is a perfectly reasonable argument. In other words, I might respect your self-identity, but that doesn't mean I have to do/believe in/accept your belief system out of decency. 

Christianity informs every aspect of a Christian's life.

I am a Christian

Therefore you should "do/believe in/accept" Christianity out of common decency and respect for your fellow man. 

It just doesn't work. One can respect someone's self-image or not, while yet dismissing their belief system as balderdash. As famed attorney Clarence Darrow once pleaded, Hate the sin but not the sinner. 

It is common decency to do/believe in/accept X

Therefore X is good/true  Non-sequitur.  Believing anything whether out of common decency or whatever is not veridical proof of truth, goodness, or even a compelling reason to believe.

I think it's prejudicial language . Just like "the fool says in his heart (whatever)", here is "Only rude people don't do/believe/accept X" and the opposite is "Do you accept X? Good (pat, pat), at least YOU're decent!"

Thomas Paine's "Common Sense"? If so, I totally agree.