Fandom is defined by meticulous care and really giving a shit and having strong opinions and arguing. It’s a community with opinions. If they were in a room they’d be fist-fighting some of the time. You know, the Sam-girls and the Dean-girls would be like pulling each other’s hair, some of them! That goes with the territory, and it’s also immensely rewarding because they’re giving a shit and they’re paying attention
Since the beginning of the show, some fans have had a strong allegiance to either Sam or Dean. Sam girls are devoted to Sam and will protest any perceived slights against his character by other fans, Dean or the writers. A common complaint is that the story is too focused on Dean at Sam's cost. Dean Girls of course protest the opposite, especially with regards to Dean's mytharc, or lack therof.
The most obsessed Sam Girls are known as ESGs - Extreme Sam Girls. Similarly, the most obsessed Dean Girls are known as EDGs - Extreme Dean Girls.
Becky Rosen is a canonical manifestation of an Extreme Sam Girl, being intensely focused on Sam and dismissive of Dean. She is sometimes thought to be a negative representation of any fangirl, and as such, has been a bit of a bone of contention within the fandom.