The outrage did indeed cut across political lines and not, I think, only because there were children in the captive audience. There were people of every sort in the audience. In an increasingly fractured society in which so many forms of entertainment are niche-marketed, the Super Bowl is one of the few truly national events - one of the few events which Americans of every race, creed and generation look forward to watching together (even if they don't know the first thing about football). Such events are rare, and therefore precious. And to be confronted by a halftime show as crass as the one in question (and I agree, the Janet event was just the icing on an already lewd cake) in the middle of a national, multi-generational, and theoretically unifying event, felt like a slap in the face to most viewers possessed of taste, even those who would never in a million years consider themselves "conservative."
(As a side note, I think the main difference between football and gladiatorial games is the former's notable lack of on-field homicides. Though it may only be a matter of time :)