Right and wrong aren't viable concepts in a relationship. It's not a competition, and there is no winner or loser.

Ross was not thinking about Rachel's feelings when he slept with the copy girl. That's why it hurt. She grows as a person, but it still feels like a betrayal, and it takes her accepting some of the responsibility for taking Ross for granted. She's still hurt, and so is he, but they (eventually) reconcile because it's TV and viewers want a happy ending.

In reality, people grow and change and compromise, and most people know more than 5 other people so there isn't "the one" in the end. You find someone that supports you, inspires you, makes you better, and is someone for whom you can do the same. Those are the best relationships. Sex is a part of that, but everyone has different perceptions of sex, how important it is, what dangly bits fit into which holes, and ideally how many concurrent participants are involve. Being compatible means understanding your partner's ideas on the subject, and I think Ross knew the limits of Rachel's tolerance, given that he lied about his decisions.