Is there a word for this?

It depends on why the other person should not be in the relationship. One word I can think of is an affair...

 

If the person need not to be there is married, then this is simply cheating. So, depend on the circumstances.

 

Sounds like something out of a Harlequin Romance novel.

Or, Robin Hood and Marian?
Lancelot and Guinevere?

Maybe "courtly love" fits the bill.

Or "erotomania", if you are kind of nutty.

 

it's called Hemingway's "a sun also rises"!

 

Calling it a one-sided affair or one-sided relationship seems to cover it. :)

 

Star-crossed?
In 'Romeo and Juliet' Shakespeare uses this term, meaning their love is affected by external forces, an astrologically malign star, more perhaps, destined to tragedy. This might apply as much to one as to both being afflicted to the same end. This has also, for example, described an Arabic story, 'Layla and Majnun', where in a common version, the problem lies only with Majnun's circumstances.

 

Are you really looking for a single, French word to describe the predicament? If it doesn't have to be French, will you accept a phrase, or do you literally mean a word?

 

L'Amour Interdit: Forbidden Love.

 

Nice work!

 

Something like,

LA-moor AHN-tair-dee

 

Nice. I see I missed some of the requirements

 

Not really. You mentioned "tradegy" in your previous post. The OP gives no references, so we can not be sure as to what manner he would be using the term. Tragic love would also be a desciption as the storyline could be described that way. Affair of the heart, affaire de coeur, courtly love, illicit affair.

To me fordidden love has connotations of the fordidden fruit, the original sin, which leads to the tragedy that befals the human race, so we have gone full circle.

 

Thanks 256bits, and really nice circular thoughts.

 

Awww i OP i can think of one such situation where the issue was that one set of parents was deeply cultural/religious and forbid their son from marrying the love of his life; he hid it for a while, but eventually had to give in to the arranged marriage his parents chose for him.
It happens a lot. Not sure what situation you're referring to but i love the aforementioned french word :-)