It seems weird that the Germans only had one mobilization plan and it involved invading France regardless of what the war was. But I guess if everyone says so.
The story of the Schlieffen Plan is something like this (and I say something like this because there has been eons of semi-pointless debate over this in academic circles).
In the beginning, there was Germany. And Germany looked at itself and saw that it was good. And Germany celebrated this by eating some sausage and shooting a few Namibians who probably didn't deserve it but hey ho they didn't have silly spikes on their hats so they were probably evil or something. Then Germany looked outwards, and saw its neighbours, and saw that they were not good, because they were not Germany. And Germany grew cross and grumpy.
Initially, German war planning in the pre-1914 period envisaged a few different plans, dealing with subjects like an attack on Russia or France in the case of one or both being engaged. There was some flexibility in this.
However, there were also issues. It became increasingly clear that war could not remain limited, as for one power to abandon another to its fate would surely tip the balance of power irrevocably against said power as well as creating diplomatic difficulties. Thus the likelihood of a war with just one, France or Russia, decreased dramatically. From there, two problems arise that lead to fixation on a France first plan.
The first, and this needs to be pointed out, is that drawing up mobilisation plans is a VERY complex, time consuming, and costly business. This isn't something that can be done on a whim. It takes months to do this just for your own army, factor in the need to cooperate with your allies (for example, travel arrangements for Italian forces being sent to the Rhine were discussed over a period of years with agreements varying greatly), and you're looking at a very lengthy process. Moreover, you have the problem that if you have multiple plans, you have scope for confusion at a time when every second counts. Better to have one plan and settle on it from this perspective.
Secondly, from the German perspective, France was the only way to strike. There are a few basic calculations in play here. Firstly, Germany needs a quick and decisive victory. This is partly because otherwise she may be overwhelmed by superior resources, and partly due to the almost single-minded fixation German thinking placed on winning a Cannae style victory (war had become an art in a literal sense, winning a stunning perfect victory being like presenting the Mona Lisa of battles). With this need in mind, we can rule out Russia as an offensive option, because there is no hope of a decisive victory against a power who can so readily trade space and manpower for time. The result would be a French attack in the Rhineland whilst Germany was struggling in the East. In contrast, the Russians will be slower to mobilise than the French (contrary to popular belief this assumption was correct, the Russians however got around the mobilisation problems by attacking with standing forces) and thus Germany might defeat France in time to rush East and defeat the Russians. Additionally, the violation of Belgian neutrality is not seen as an issue, Britain is thought unlikely to intervene and insignificant in a short war, and the Austro-Hungarians should be able to handle some of the Russian forces as well as the Serbians thus buying Germany time.
Of course, this all turned out to be nonsense. The German vision of a modern Cannae was undermined by various factors the most major of which was an operational overview which placed unrealistic demands on the troops involved. But judged on the basis of achieving a decisive victory, it was still the best option.
