It's considered sacred. It's asked not to be shared and depicted for respect of those who believe and find the ceremonies sacred.
That's the short answer.
That's also an answer that doesn't say anything. Let's phrase it a little more obviously: why is the act of filming a simulation of a temple ceremony (i.e. not an actual ceremony) offensive? Is it really that upsetting that someone who's not of the faith knows what happens inside the temple? They still can't participate in the actual ceremony nor are they able to view the ceremony itself being carried out in the particular place of worship.
Saying it's sacred isn't really an answer, it's a conclusion. I want to know specifically why the simulated depiction of a ceremony is so offensive. Even better would be a defense of why it's a "million times more offensive" than burning the book of Mormon. That comparative claim is interesting to me given that the latter is at least representative of the physical and metaphorical destruction of the religion and its ideas.
I regard the idea that Big Love should be constrained from simulating the ceremony (which, having seen the episode in question, there were valid artistic reasons for doing so) as just as ridiculous as South Park being prevented from depicting Muhammed.