Is it okay to pursue a girl already in a relationship?

(Disclaimer: This has nothing to do with my running thread. We're happy and are going out again this weekend )

I've currently on my first watch of The Office. Early season 4, so the show is still exploring the aftermath of Pam and Jim finally getting together. I've always been interested in romance (500 Days of Summer and Annie Hall being among some of my favorite movies) so this love story line is in my wheelhouse. I keep finding myself going back to the scene where Jim finally tells Pam how he feels. And damn, what a scene it is. No cheesy music. Just Jim manning up and letting it fly. We see the conflict fill Pam's face as she lets Jim down. Jim sheds some tears and sulks away. The end? Nope. After a couple of minutes on Michael Scott's weird love triangle we come back to Pam and Jim in the office. BAM!. Jim lays one of the most forceful, yet respectful kisses ever on Pam. This show isn't messing around with this story.

Would this ever work in real life in this situation? Is Pam, an engaged woman who has dated this man for years, in too deep? A couple of prefaces. Both the viewers and Jim know that Pam isn't invested in relationship with Roy. Everyone , including Pam, knows that she is better off not being in this relationship. Does this make Jim's actions better? The two have flirted for years and might be closer than the average man who may want to pursue a woman in a relationship. Does this impact Jim's decision, thinking she would be less likely to freak out?

Obviously not talking about going after a married woman. I believe most people would consider this hands off. Also begs the question, what does 'pursue' even mean? Was Jim pursuing Pam, or just laying it out there for her to weigh. Did the kiss go to far? Would love to hear some stories on this.