When a guy crosses the Rubicon with a girl, it is basically the point where it is impossible for them to be just friends. It is the "at bat" portion of the baseball game, and the next step for the guy is to get to first base or strike out. In other words, the guy either ends up in a relationship with the girl, or he fucks it up and they never speak to each other again. But, either way, it's far better than being in the Friend Zone, and crossing the Rubicon with a hot girl is always something to be admired.
Alex: But she was so nice to us, and we all had such a fun time hanging out, but she ends up fucking Tristan? He was also there the whole time and they never spoke. I thought she hated him!
Mark: Yeah, Tristan's an asshole. But you know, girls like assholes.