Certainly "The Light in the Piazza", but some of the Rodgers and Hammerstein shows ("South Pacific" and "Carousel") in particular are rather romantic. But "The Light in the Piazza" has no other major theme but love.
'Light in the Piazza' is an obvious one, but the first things that came to mind for me were 'South Pacific' and 'Rent'. Both contain multiple, compex love stories around significant social issues.
"It does me no injury for my neighbour to say there are 20 gods or no god. It neither picks my pocket, nor breaks my leg."
-- Thomas Jefferson
Piazza, Rent and I find The Wedding Singer very romantic.
"People have their opinions and that doesn't mean that their opinions are wrong or right. I just take it with a grain of salt because opinions are like as*holes, everyone has one".
-Felicia Finley-
Defintily South Pacific. Carousel Comes in at a close second.
<------ Me and my friends with patti Lupone at my friends afterparty for her concert with audra mcdonald during the summer of 2007.
"I am sorry but it is an unjust world and virtue is only triumphant in theatricle performances" The Mikado
Though certainly not the most romantic, I find The Pajama Game revival (with that amazing chemistry between Harry and Kelli) to be a fantastic story of love. I mean, you can't buy that chemistry between them, and that's the biggest thing you need in a love story.
Also, I'm surprised nobody has mentioned West Side Story. Again, it's not the most romantic, but that last scene? Fantastic!
Back, he spurred like a madman, shrieking a curse to the sky. With the white road smoking behind him and his rapier brandished high! Blood-red were the spurs i' the golden noon; wine-red was his velvet coat, when they shot him down on the highway, down like a dog on the highway, and he lay in his blood on the highway, with the bunch of lace at his throat...
(The Highwayman. Sung by: Loreena McKennitt)