Imagine person C was the same as person D.
Person D just has a different perspective.
Scenario 1)
Person D lives in another country, such as India, where it's pretty standard to marry someone for life? (my speculation) And 'dating around' / having many relationships isn't common.
Scenario 2--not a carryover from #1) (Person C is still person D-- who loves person A)
Person C meets person A and has a relationship with them...Person D does not know person B. D has never even heard of them. C's relationship goes sour with person A and A ends up later on with person B. D is like, "that's the new guy/gal" and is relatively uninterested/hopes person A is happy.
Scenario 3--also not a carryover from #1 or #2) (Person C is still person D)
C meets person A and D also KNOWS Person B -- but D does not care whether this person loves them or not. D sees their relationship (A & C's) as a separate part of both lives, and D completely trusts person A. But again... (as above) it goes sour. And they end up with person B. But again, D is like, "they finally got together" and is relatively uninterested/hopes person A is happy.
My interpretation of this is
1) Multiple relationships is western culture only (US, UK, some latin countries and western europe)...(for the most part...maybe some other cultures too, I'm not expert) and there are other possibilities. There are other norms as well.
2) If Person C didn't know Person B then Person C could not 'fear' or have suspicion about their(person B's) involvment with Person A. If this is happening or has ever happened to or with Person C, perhaps there are some power issues in the relationships that Person C has with the opposite sex (but probably everyone). They may even be occurring in the present.
3) Notice how Person D can also just see them as people and not 'get inside their heads' and put all thoughts about them getting together before or after the current relationship (what does that have to do with the present anyway?).
Finally, Person C may (if Person C is even real) have his or her ideals, and I respect that. Perhaps person C would be happier if they could remain in the present, enjoy the relationship they have/would have hypothetically with person A, and that relationship would probably not end (just a random guess, there are almost always other factors involved) without the worry, jealousy, envy, inferiority complex, or fear of abandonment that comes with paying (in this case) too much attention to what other people have.