Rebecca & Nathan are so great. Altura would be a lovely spot for Valentines Day.

Sutra. Every person I've ever recommended it to has loved it.

Lark is my go-to romantic restaurant, extra romantic if they seat you at the corner table in the back. I'll also +1 the Altura recommendation. Been there for a number of "special occasion" meals (and a few "what the hell, why not" meals) and they never disappoint.
Oh, and counter seating at Spinasse can be very romantic, especially over a shared bowl of tajarin with sage butter. Awwwwww yeah.

Carmelita was our favorite place for Valentine's Day when my wife and I were still in Seattle. Too bad they're closed now, but it was very nice at the time. Pre fixe menu, romantic poetry, special ambient lighting used only on Feb. 14, top notch service... I miss that place.