Historical Reenactors (and I mean Historical Re-enactors, not blanket SCA/ Renn Faire types) grapple with this question daily.
The general attitude amongst them is that in order to respectfully portray a person or persons from the past is to remember that the clothes they wear and the things they do represent people.
Real live flesh-and-blood, thinking, feeling, people.
Really hammering that point into your own mind will bring a deep sense of humbling perspective, and while it might not completely legitimize your hobby, it may allow you to more fully accept what you're doing and think about what it means to you.
Another thing they do is research the everliving shit out of who/ when they portray.
That doesn't mean "I read one book with pictures" or even "I read 10 books and wrote a paper with citations", it means "My research is not secondary, but primary to my reasons for reenactment. My knowledge will grow and mature as I mature, and I will make it a corner stone to my sense of self for the rest of my life".
You don't have to go that far for airsoft, and many people who do that probably don't think about it in those terms.
You can put together a completely generic 'tactical' loadout from commercially available gear and think about nothing more than the efficiency of the game itself, and many (younger) military/ LEOs won't even bat an eyelash. Things are different now. The vast majority of our gear isn't even surplus anymore.
However: If you do want to go out and replicate the 'look' and persona of a certain military unit, I personally believe that it's your responsibility to ensure that it is accurate, neat, best represents the most correct picture of that unit (this includes physical fitness and appearance), and that you think of it on the same terms as a historical re-enactor does.
HRs often talk about being 'in character'. To some of them, that means they are a professional or amateur actor and are actively portraying someone as best they can. To all of them, this means not doing shit that their subject wouldn't do. Example: picking nose, profanity in public, lack of profanity in public, mishandling weapons, undue horseplay, being a twat etc.
This is largely subjective, but still subject to the fact that a military uniform is someone else's skin. Don't defile it.
And don't wear rank/ insignia/ patches you haven't earned. This is a bit looser in the context of HR, but there is a mile-wide line in the sand for it in airsoft.
No matter how main stream airsoft gets, it will always be on the fringe. There are people who think that wearing combat boots with normal clothes make you a horrible person.
Most reasonable people will see you dressed up and will range in opinion from "Awesome" to "Well, that's... different, but to each their own". If you stay generic or derive from fictional sources (I wear a RAINBOW patch) people within and without the community won't scrutinize you, but the second you're identifiable as 'US Ranger Fallujah circa 2006', '101st Airborne circa 1945', or 'PPCLI circa 1989', you are putting yourself under scrutiny. If you're a decent guy, you'll likely get away with it, but if you're wearing unit markers, boards and other insignia you are in serious shit, or not invited back.
This is one of the great reasons why teams invent their own patches and markers.
This is all pretty common sense stuff, but a lot of people just don't get it.
If you're worried about what you might run into, just buy modern, generic, new stuff and set it up in a way you find efficient and attractive. SEALs don't have a copyright on Sigs or 6094s.