Or, to put it another way, you have to evaluate the rationale behind a rule, you can't simply follow it and say, "well it's written down so that's what we have to do". And if you look at the rationale behind the 14th amendment, it's pretty clear what it is. The goal is to create legal equality for all citizens, it's a fairly straightforward and admirable motivation, especially in a country founded on the principle that all people are created, and therefore should be treated, equal. The motivation behind the second amendment is more complicated. Apparently the reason why the right of the people to keep and bear arms should not be infringed is because a well regulated militia is necessary to the security of a free state. So the key to understanding the motivation behind the second amendment is in understanding the link drawn between private gun ownership and a well regulated militia. I believe, and I could be wrong about this, so if my facts are way off, someone OTHER THAN TMW, please do correct me, that back in 1791 the US military was much more loosely organized than today, and the idea of individual citizens rising up and defending their homeland was a much more feasible idea. However since the assembly line had not yet been developed, guns could not be mass produced and instead were much rarer and much harder to come by, so individual citizens joining defensive militias would be expected to provide their own firearms. Nowadays military technology has progressed to the point that the kinds of firearms that people do have access to would be of little to no use against an invading force armed with equipment like rocket propelled grenades, armoured cars, tanks and aircraft. This makes the idea of private citizens rising up against an invading force a completely futile effort, your Red Dawn fantasies aside. Furthermore, the advent of mass production has made the concept of providing your own firearm for military service completely obsolete as now firearms have become relatively inexpensive and easy to produce. Consequently the army can provide weapons to all of its members regardless of whether or not they own their own guns.
And that is why it is not a double standard.
And in response to your comment that the PRC will definitely invade, it'll just take longer than a decade...that's a very foolish thing to say. You don't know what global geopolitics will look like 10 years down the road. I don't either; no one does. I can't say it with any kind of certainty, but I will say this: over the past several decades, since Nixon's visit, China has been pretty consistently moving towards increasing openness and co-operation with the West. This trend has done nothing, if not accelerate over the past 10 years, and this kind of social change tends to have a certain inertia, I expect this trend to continue until eventually relations become so close that China and a number of western powers, probably including the US become full allies. I'm not saying it's impossible for an invasion to occur, but this conciliatory trend would very, very, very likely not only stop, but actually begin to reverse before a Chinese invasion became a real concern. In other words, if it were going to happen we'd have lots of warning. I know what you'll say "they wont' telegraph their moves so obviously etc etc etc." But Diplomacy =/= life. That's simply not how things work in the real world, because the longer that Sino-American relations remain amicable, the more severe the consequences of invading will become.