Damn, I was about to put UF/Zooropa/Passengers before you replied.
The ethereal sounds are just perfect for creating a romantic atmosphere. Plus a lot of the ambience and atmospherics used on Zooropa and Passengers in particular are for songs concerning love.
The main examples are:
The First Time & The Wanderer - both love songs
Slug, Your Blue Room and Beach Sequence are all about incidents about love while Plot 180 is about a sex scene.
ATYCLB also deserves a mention. It could be played for a romantic evening in as a lot of its songs are about love - e.g.Elevation, Walk On, Kite, Wild Honey and The Ground Beneath Her Feet. The main one, and possibly U2's best love song ever is In A Little While.
If, however, you are like me, miserable and angry about love then you're best listening to Achtung Baby or Pop which both deal with love in crisis. The main examples are One, UTEOTW, WGRYWH, So Cruel (the main one), The Fly, Mysterious Ways, Love is Blindness, Do You Feel Loved?.
Then again, Pop does have one song on it that would set the atmosphere for a romantic evening in, If You Wear That Velvet Dress. I find that that song bears most influence from the preceding Passengers project. It is almost like Plot 180 but with lyrics and a melody and song structure.