Here are the songs I listened to:
"Love Theme" (Ennio Morricone)
"Rose" (Titanic, James Horner)
"Theme from Love Story" (you know, that descending minor sixth
!)
"Uncahined melody" (Ghost)
"All I ask of you" (Phantom of the opera, Webber)
"Moon River"
"Out of Africa" (John Williams)
"To the stars" (Randy Edelman)
"Across the stars" (Williams)
"Theme from ice castles"
"Can you read my mind?" (Superman, Maureen McGovern)
"Marion's theme" (Raiders of the lost ark, Williams)
And a few others
Some of them weren't used as love songs, but they could easily have been, I think.
What I love from the first one I listed is the melody exchange between clarinet, flute, french horn and strings. I think it's useful for repeating the theme whitout tiring the ear. The accompaniment also changed.
I agree with you that such a melody need to be catchy, so we can't easily put romance in a movie simply by starting again the "romance song". That's in part why "Theme from Love story" works so well, the first interval being a minor sixth easily recognizable.
Last thing I realized with "To the stars" (heard in Dragonheart), is that the theme in itself can develop... hard to explain, but when you listen to it, it's pretty clear I think.
So I'm going to try writing some melodies having those characteristics... soon.