Fallacy pertaining a parallel to the past

Whenever someone raises a concern or complains about something like how vulgar some songs these days are, it is not uncommon for someone else to chip in and point out that similar comments were made about the Beatles and other bands from the 20th century. Since no one these days would consider those lyrics vulgar, you are wrong about songs today. What exact fallacy is being committed here?

It would probably follow this logical form:

X is considered to be bad.

Y was also once considered to be bad, but not anymore.

X is therefore not bad.

It's maybe something to do with the fact that, because people in the past are wrong by today's standards , you are also wrong.

I hope this makes a bit of sense ;)

Welcome Mr. Brinstar,

Very good question that leads to some heavy philosophic musings. For example, when Elvis was rockin' his hips, was that "bad" and "vulgar," or were these claims overreactions of a segment of the population? If the former, are they no longer "bad" and "vulgar" or did society's standards stray from what is good and right? Are such lyrics and moves objectively bad or good? Who says?

On the one hand, arguments such as the one you references do point out changing societal norms which suggests that such lyrics were never "bad" to begin with; they were just outside societal norms. On the other hand, one could argue that our changing norms is due to a "moral decline."

Using your example, I would say there is no fallacy, because comparing today's music with music in the past is a strong analogy. However, if we said something such as "The Beatles' music used to be seen as morally problematic but not anymore, rape is currently seen as morally problematic, so rape is not morally problematic," this would be a weak analogy.

The logical form you laid out is only problematic when X and Y are different enough where the conclusion that X is not bad is no longer reasonable.

Though stretching it, this could be a form of Tu Quoque (ad hominem) "whatboutism"  if phrased in a different way. 

X:  Music lyrics today are vulgar and crass

Y:  Every generation says the same thing about the younger generation's music.

Z: Therefore today's music lyrics are not vulgar and crass.

Apart from being an ad populum/hasty generalization, the mere suggestion that today's music is not vulgar and crass because other generations have had the same complaint is not a valid argument. 

Perhaps I'm being overly sticky about definitions, but it seems that using somewhat global and subjective generalizations (e.g., "good" or "bad") can easily weaken an argument ... not necessarily making it false but certainly making it weaker.

Rewording the argument to:

• X is considered vulgar by today's standards
• Y was considered vulgar by the standards 50 years ago but is no longer considered vulgar by today's standards
• Therefore, X is not vulgar today

makes it sound less convincing -- although the content is essentially the same.  

The most logical conclusion following from the initial two re-worded statements above would seem to be something like Therefore, (even though it is considered vulgar by today's standards) X may not be considered vulgar in the future .  

The best conclusion one could draw from would be that societal standards have changed in the past and are likely to continue changing into the future so it's reasonable to think that what is considered vulgar today might not be thought of in the same way in the future.