I have seen NAMBLA brought up a lot, particularly in homosexual debates and same sex marriage debates. So, as far as the "leftist group" being "ignored" - I don't think so. I think that is only the perception - they aren't in the msm as much because they haven't committed any crimes as an organization, recently. In fact, when that horrible crime was committed, and NAMBLA materials were found in the apartment of one of the suspects they were very much in the news.
The difference though - as I see it is this.
NAMBLA is a group MUCH MUCH smaller then the FLDS (NAMBLA's membership has never numbered more then about a thousand) who do not appear to act on their beliefs much. When members have committed crimes: those crimes are prosecuted and are out in the media.
If NAMBLA - as an organization (protected under freedom of religion) were to create a compound and force relationships like this cult has done - then you better believe it would be in the news every bit as much. But they haven't - they are a small isolated group of perverts who apparently do little beyond existing in dark scummy corners. They certainly don't seem to be holding children against their will in compounds.
In terms of the whole issue of polygamy - I could care less. I'm with you on that. What consenting adults CHOOSE to do of their own free will is up to them - whether it's one man, one woman; one man, many women; many men, one woman; man and man; woman and woman - or any combination there of....who cares?
What rational, logical reason is there not to have polygamy that could not also be used against "traditional" marriages as defined today?