I think Om is really sexy/romantic. They play great electronic lounge music. I had a terrific cheese plate with wine paring on my last visit.
Upstairs on the Square has terrific desserts and is somewhat romantic.
Rialto in the Charles Hotel re-opened last week. Desserts were pretty great before they shut down for renovations. I haven't seen the new space but I imagine it's terrific. The views in that room are lovely and I've always had superb food and service. I was reading that they added 30 new wines to their list. Makes me happy. :)
Finale is another option. Swank desserts by candle light. I'm not a fan of the view though...