When completely ignoring the gay marriage thing, this thread actually has some life left in it..
Say that I'm in high school, I have brilliant grades for maths, physics, chemistry, biology, I'm planning to be a scientist, I have the brains required, I'll probably cure cancer and win a nobel prize, etc etc etc
But I suck at french. And german. (both required subjects here in Holland).
Now, I normally study JUST the correct amount to pass the tests, and thus get my diploma.
But on one very unfortunate day, both my french and german teachers decide to give a surprise test on a subject that I was supposed to study, but which I hadn't studied yet since the scheduled exam was in 2 months and I was busy reading issues of Science and Nature. (and yes, they actually give surprise tests here..)
Now, I'm going to get a horrible grade for this test. One that can't be compensated anymore. So I'll fail the class, I won't get my diploma, I'll have to redo the entire year, and the cure for cancer will have to wait..
And that all because of French and German, two languages I'm not going to use. No one in my future career is going to care if I don't speak a single word of French of German.
...If you ask me, cheating on the test in this situation would be morally acceptable, if not the best course of action.
Too long; didn't read-version: is cheating also bad if you're never going to have to use the knowledge/skills tested? I doubt it actually.
edit: note the mention of skills in the tl;dr version. For example, my french/german grade on my diploma says SOMETHING, namely that I'm good at memorising stuff. A valuable skill. But if pure testing of memorisation isn't the goal (e.g., if you've succesfully remembered 10X as much for a previous test), then cheating on a "learn stuff you're never going to use!"-test can be excused I think.