When I think of a romantic restaurant, I do not think of a brightly lit room with wooden chairs and resonant floors. Some time ago, there was a French restaurant not far from Ghirardelli Square with fabrics on the walls, booths, and excellent food. Service was discreet. I took my girlfriend, and it was memorable. We also ate at Lutèce while Soltner was running the show, and it was stunning—and romantic.
Do any such restaurants remain in San Francisco? And might they tolerate my vegan ways?
The only vegan restaurant that comes to mind which has an inkling of elegance would be Millennium.
Saha serves some great vegan, but they aren't quite as elegant inside -- although I like their food more.
There are two others besides Millenium that will fit the bill. Greens at Ft. Mason will happily accommodate vegans, and The Richmond in the Inner Richmond will do a very nice vegan prix fixe menu with wine pairings (I did both of these when the wife and I were vegan) if you give them a couple of days notice.
Another place that might be able to accommodate with advance notice is Sons and Daughters. They are more than happy to work with dietary restrictions and they made my wife some amazing gluten free breads for our meal.
From photos, Keiko à Nob Hill looks right, and I've seen reports from people who were happy with special-request vegan menus.