@glitterbag,
You may not have been in bars that have developed a clientele of regulars on Sundays during football seasons.
Sure there's lots of cheering and cursing and, for the most part, good natured ribbing between the fans of different teams, but there's also a lot of "analytics" going on (sometime too much).
With so many TVs on the walls and plenty of down time, there are always opportunities to offer opinions on every aspect of the game, in any game.
Wherever I've lived, I've gone to the same bar every week to watch the games, and it's always been the same. A lot of people watching football, cheering their team, cursing their team, kidding around and talking football.
You're absolutely right about how rabid some rivalries are but it's always seemed to me that this is more pronounced at the location of the game. I think it may have something to do with tribalism and a sense of territoriality. I'm sure the drinking helps but home team fans seem to, generally, have less tolerance for the fans of the opposing team who invade their turf, than if members of both groups were in a bar together. It doesn't help either that the visiting fans who make the most noise are often jack-asses. You have to have had a lot to drink and be pretty aggressive to draw attention to yourself in enemy territory.
If I'm a visiting fan, I'll cheer for my team's successes but I keep quiet about the home team's failures. Unless in a place like Philly where they boo Santa Clause and have jail cells in the stadium, this strategy usually works. Rubbing it in with a home fan when their team screws up or just plain sucks, is asking for a fight.
I'll admit I'm a fan who not only loves my teams, but hates others. I enjoy hating the Redsox, the Flyers, the Cowboys and the Lakers. It's a harmless way to hate, and probably emotionally healthy in a strange way. I don't want their players to die in plane crashes of course, I just want them to always lose, and badly, but with rare exception, if one of them ends up on one of my teams, they're deserving of love.
Sport is great on many levels, and there is no way a game like soccer could be loved by so many people if it wasn't a great one. It may eventually catch on really big here, but it's no big deal if it doesn't. I'm sure the people who make money on the game want to see the American soccer market grow, but if it doesn't, it isn't going to hurt the game. It doesn't need us.