All credit to maisie7 on fanforum (I would post a link, but it’s fanforum…)
Their dialogues in their break up scenes:
THE INCIDENT (Juliet & James)
JAMES: I need you to tell me where all this is coming from.
I mean, one minute, you're leading the great sub escape, And now you're on board with blowing up the damn island? I got a right to know why you changed your mind.
JULIET: I changed my mind when I saw you look at her. Don't. don't.
JAMES: I don't care who I looked at. I'm with you.
JULIET: And you would stay with me forever if I let you, And that is why I will always love you.
JULIET: what we had, it was just for a little while. And just because we love each other, It doesn't mean that we're meant to be together.
I mean, maybe we were never supposed to be together.
So if Jack can make it that-- that none of you ever come here, then he should.
JAMES: Why you doing this, Juliet?
JULIET: I...If I never meet you, then I never have to lose you.
FLASHES BEFORE YOUR EYES (Desmond & Penny)
DESMOND: How can I? I - I can't look after you. I haven't got a job. I don't have any -- I can't even afford 5 quid for a bloody photograph. You deserve someone better.
PENELOPE: I know what I deserve. I chose to be with you.
I love you.
DESMOND: Love's not enough.
Being a good man is not enough.
PENELOPE: What's this about, Des? Where's this coming from?
DESMOND: It's all happening too soon -- you moving in. You're painting rooms; you're changing things. I don't even like red. Why would you leave your flat, your expensive flat...
PENELOPE: Don't do that. Don't you pretend you don't care. And don't you dare rewrite history.
I left my expensive flat because you were too proud to live there, remember. If you want me to go -- if you want me to leave then don't make this about what I do or don't deserve. And have the decency to admit that you're doing this because you're a coward.
DESMOND: I'm sorry, Pen, but this – we're not supposed to be together.
Other things that paralleled Despenny and Suliet:
1) Desmond & Penny have been together for 3 years before this break-up, just like Juliet & Sawyer.
2) Desmond spent 3 years on the island and then he found Penny and they are together for 3 years now. Juliet spent 3 years on the island and she and James have been together for 3 years.
3) Before leaving Penny, Desmond met Eloise Hawking and she told him that him and Penny were not supposed to be together. Before the fight with Sawyer, there were a flashback where Juliet's mom told little Juliet that even if two people love each other, it doesn't mean they are supposed to be together.
4) In FURTHER INSTRUCTIONS, Desmond tells Hurley "The failsafe key must have detonated the electromagnetic anomaly", Juliet in THE INCIDENT detonated the bomb in the same hatch.
5) In LIVE TOGETHER, DIE ALONE, there is a white flash after turning that key, just like the one in the last scene of THE INCIDENT.
6) After turned the key in the hatch, Desmond goes back to his old life off the island and then he starts to remember things from his period on the island. He talks with his friend in the pub about time travel. Juliet and all the people who were left behind travelled through time.
7. The title of season 2 finale is Live Together, Die Alone
where Desmond blew up the hatch. And in the season 5 finale, Juliet answered “Live Together, die alone" to Sawyer. Then she later on blew up the jughead on the same hatch Desmond detonated 30 years later.
Desmond was going to propose to Penny, but after Eloise Hawking stuck her nose in his business, he decided against it and broke off with penny. He then threw the engagement ring he bought for Penny into the river.
Desmond and Penny are so epic, though, that the ring was eventually given to Penny(after they were reunited after years of separation) and they got married with both of them wearing rings.
In the Flash Sideways, Desmond finds Penny and asks her out on a coffee date, which she accepts.
James and Juliet had lived together for three years in 1970s Dharma times. They "played house" and confessed their love to each other. James had bought an engagement ring for Juliet and wanted to marry Juliet, but before he could, she died in The Incident.
Heartbroken, James throws the engagement ring (that survived a flash forward thirty years in time) into the ocean a la Desmond.
Before Juliet dies, she experiences what we believe to be the FlashSideways universe and in that moment, has a conversation about going dutch for coffee with someone. We can assume it to be James, which makes James and Juliet very Desmond and Penny.