I am a lady who turned 40 in SF (not too long ago, mind you), so I'll share my experience. I also lived there many years and had the opportunity to try most fine dining restaurants in the city.
Dining Room: it's all that had been said, but the ambiance is a tad bit too "old". Your fellow diners will be, too. IME. The place is too quiet, where diners on the next table can hear you, unless you whisper.

Romantic places:
- The Campton Place: small dining room, but food and service are excellent (had my bday there last year)
- Masa's: love their table for 2, where the couple gets to side by side. A little "last supperish", but it makes (private) conversation easier.
- Silks at the Mandarin Oriental
- French Laundry: the Holy Grail. Obviously, it's hard to get a table... good luck with not get a busy sign when you call. If you are willing to go there for lunch, or on a weeknight, your chances are much higher, once you actually get to speak to someone.
Also rans: Michael Mina, Fleur de Lys, Coi