This is, I believe, the heart of the problem: accuracy must include context. In the case of a spiritual event such as the temple ceremony, there's chronological context--what happens before and after--and doctrinal context--the underlying role of the temple in Mormon beliefs--and spiritual context--the linking up of new revelation with prior revelation.
The first two are hard to provide. The last is impossible. Consider this: If I tell a joke about Mormons to you, how much of that joke do you stand to lose if you have no knowledge or experience of Mormons? I might be perfectly, flawlessly accurate in my retelling, but you still stand to lose some or all of it.
The temple ceremony is carefully crafted metaphor intended to transcend mankind's limited language. In that sense, it's like poetry, or jokes, or art, all of which also requires a contextual framework in order to understand it properly. This is why we require new members to wait a year after baptism before they visit the temple: they need to accumulate a whole new set of spiritual experiences so that the ceremonies will make sense.
This would be reason alone to not show this on HBO, but it gets worse: by putting this as part of the Big Love series, the creators are not just showing it out of context, but are giving it a different context within the storyline of the show. This then further distorts the ceremony's original meanings and purposes, no matter how accurately it is portrayed.