Translation - French-English - L’amour est si pur qu’il ne se souillera pas de l’érotisme...

bir zamanlar = once upon a time

Hi merdogan,

"once upon a time" is a classical expression used when writing a tale, just to start narrating something that happened in the past. It's a way to place events in the past. See what I mean?
Children's storybooks usually start that way.
It's not used for statements like this.

It's so noble that it will not have been reduced...

Hi jollyo,

That future perfect you suggest is not what is used in the original.

"it might not be reduced to the grave." is a bit too literal, and because of that it's meaning not very clear.
I would suggest instead "it will not diminish until the grave."