Japan cannot re-militarize. It's constitutionally impossible. Even if they could I think there's more going on here. There's a lot of underlying conflict in Japanese culture that isn't immediately visible. This kind of conflict spans thousands of years of cultural history; the cultural thing to do is to hide your true feelings in the name of harmonious relationships[1]. But the outcome doesn't seem to be that conflict is truly stopped just deferred.
In the case of Japan birthrates have been a subject of concern for at least the last two decades. There are many potential contributing factors, not the least of which is the enormous amount of stress on the average family and the exorbitant cost of living in a megalopolis such as Tokyo; the stress of working long hours and having high expenses is enough to prevent people from having kids and even having a relationship.
Although geopolitically Japan is generally a peaceful society de facto I would argue that that is not necessarily true culturally. And even in the context of geopolitics there is a fair amount of drama in Asia that involves Japan, e.g. between the South Koreans, China, Taiwan, and Japan.
Actually the reason why Japan cannot re-militarize is that it already has done so decades ago. Its military may be called "self defence forces" rather than "armed forces", but it really is a well-funded army of moderate size (6th highest military expenditures in the world).
The constitution MacArthur oversaw specifically forbids a military[1] and despite recent attempts to amend the constitution this has not happened. However, the limit on a military force was later interpreted as provisioning a self-defense force, ergo SDF. It was intentionally limited so as to prevent Japan from immediately rising as a military power but to not require longterm occupation of Japan by U.S. forces[2][3].
In practical terms there are only two restrictions on the Japanese military: No nuclear weapons and no aircraft carriers. Even the prohibition on aircraft carriers is a bit leaky.
Have a look at the Hyuga class "helicopter carrying destroyer": http://www.jeffhead.com/worldwideaircraftcarriers/16ddh.htm. On thing you notice is that unlike almost every other amphibious assault ship in the world the Hyuga class has an offset island, meaning you can land fixed-wing aircraft. Officially it carries 18 helicopters, but it has everything you need to carry an air wing (range, fuel, ammo, maintenance, and communications). So all they really need to to is fly off the helicopters and fly on some sort of STOL aircraft (like the F-35C) and presto! An aircraft carrier about on par with the British Invincible class.
Recently the Maritime SDF (you know, Navy) got the go-ahead for a "helicopter carrying cruiser", which would be the same thing only bigger, although still officially not an aircraft carrier.
The two you've listed and of course limits on national spending that involve the military which as a result significantly limit the growth of the SDF. Japan wants to avoid directly and blatantly invalidating their constitution; after all, it is supposed to be the fundamental set of laws that validate the state. That's why there's been such a push to amend the constitution.
In the case of Japan birthrates have been a subject of concern for at least the last two decades. There are many potential contributing factors, not the least of which is the enormous amount of stress on the average family and the exorbitant cost of living in a megalopolis such as Tokyo; the stress of working long hours and having high expenses is enough to prevent people from having kids and even having a relationship.
Although geopolitically Japan is generally a peaceful society de facto I would argue that that is not necessarily true culturally. And even in the context of geopolitics there is a fair amount of drama in Asia that involves Japan, e.g. between the South Koreans, China, Taiwan, and Japan.
[1] http://en.wikipedia.org/wiki/Honne_and_tatemae