Why does Christianity label harmless action/things/activities as immoral? Examples:Heavy Metal, Dungeons & Dragons, long hair on men, braless women, etc. Secondary question: What is the boundary that separates action from immoral action? If you cannot answer question 2 then how can you know right from wrong in terms of morality?
Note. If you claim something is immoral/wrong/bad, please say why with as much detail as possible.
Those particular examples seem more like cultural preferences/prejudices, though such cultural preferences can be commonplace in a region or nation. Now, that doesn't mean there are never any instances of those that are truly wrong. Obviously 'heavy metal' can have an enormous variety in it, and some of the songs can be genuinely a bad influence, while others are good, a good influence.
Some good, some bad, and some just neutral.
But wholesale rejecting all 'heavy metal' without any sub categories/distinctions is more a cultural prejudice I think.
The question of dress is a more complex, in that in a given culture we are not supposed to knowingly trip others into sin by dressing in a way that is sexually suggestive or overly enticing, but instead to dress modestly, but the caveat is that 'modest' is culturally conditioned so that it depends in which culture/place you are in what is locally modest and what isn't. What is perfectly modest here may be sexually provocative in another country, and vise-versa. Following the golden rule, we should try to dress in a way that is modest relative to the local culture we are in, which in America at least means it's perfectly fine to where clothing that would not be okay in some Middle Eastern nations for example.
The boundary condition is that we not be led nor lead others into breaking this rule from Christ (in this particular wording):
"So in everything, do to others what you would have them do to you, for this sums up the law..."
Notice He said 'in everything'. That's important. Example: even if my wearing some certain clothing or skin revealing of some kind may be ok here where I'm at, if I visit someplace it causes someone there at that place to trip into adultery lust, then I'd be breaking the rule above, in that I'd be mistreating the spouse of that person, by tempting their mate, while I'd not want someone to tempt my mate.
So, this rule is very powerful to help us establish the exact boundary condition all the time, in all situations. (I haven't found a situation where I can't apply it usefully.)