Don't get me wrong, I really appreciate it when a girl joins me as I do something I enjoy (in this case watching, playing, and talking about sports). Furthermore, I believe that one sign of a strong couple is when they take interest in each other's interests. That's cute. I love it.
For me, the line is crossed (and its not so much a line as it is a border between cute cool girl land and huge turn-off land) when the girl exceeds my enthusiasm. I am a big sports fan, so she has to be a crazy fanatic. And some girls are. Some girls will continuously scream and yell at the TV. Some girls (even those who have never played the game and barely understand it) will tell the player (through the TV) exactly what they should do after every play. Barf.
I know this might sound sexiest and double standard-ish, but I think it works both ways. If a guy watches a chick flick with a girl, she might (should) appreciate it. But if a guy LOVES chick flicks, never wants to watch anything else, and gets super emotional during the movies, you cannot tell me that most girls would be totally cool with that. Are you kidding me? Either the girl would be creeped out or would immediately friend zone the "dude" for lack of testosterone.
Just remember ladies, its cool when you can enjoy sports with us, just keep it under control.