I like the movie, but the characters behave despicably...
Allie and Noah may have ended up spending their lives together, but they couldn't stop themselves from running over other people in the process. The way Noah treated that war widow he was sleeping with before Allie came back into the picture was just rude, and what Allie did to her fiance was despicable.
People may say they were in love and couldn't control themselves, but they had choices. They could have stopped themselves from having sex, knowing that she was engaged to someone else. Lon absolutely did not deserve to be treated like that. He was a good guy who treated her well.
This is a big part of why Titanic will always be the better movie to me. In that movie, yes, Rose had sex with Jack while engaged to Cal, but Cal wasn't like Lon. He was an emotionally abusive narcissist, and she had made it clear that she did not love him. She was being forced to marry him, unlike Allie who actually did care for Lon. I don't fully condone what Rose did either, because it would have been better to end the engagement first, but it was much easier to root for her and Jack as a couple since they weren't trouncing all over good and innocent people in their desire to be together.
With Allie and Noah, it wasn't even a one time thing and then "oh my God, what did I just do?" They had sex over and over and over again over the course of days. And there was no mention of her fiancé until her mother showed up. Just disgusting behavior really.
The fact that Allie and Noah were "in love" doesn't excuse their behavior IMO. When she realized she still loved Noah and that he had actually written her all those letters, the RIGHT thing to do would have been to think it over and then tell her fiance she's sorry but she is going back to Noah. Not have sex with Noah multiple times before telling him.
It just makes it really hard to see these characters as sympathetic. To me they seem like two selfish and codependent people who are willing to trod all over others to get what THEY want rather than two people who are genuinely and purely in love.