Romantic, not pretentious, low key
Wife and I have one night to dine in DC and looking for romantic but not pretentious/stuffy, and won't break the bank either. Love spanish, italian, asian, and seafood. I know alot to ask for. Close to metro would be a plus but I realize I'm pushing it. Thanks!
For Italian you might want to try the small but delicious Al Crostini on U Street. The pasta is made fresh, the owner is a welcoming over the top Italianman, and the restaurant is small enough to be intimate but not cheesy. Just know that the specials tend to be waaay marked up (twice the price of most items) but otherwise it's affordable.
I thought the low lighting and chic atmosphere at Zentan was pretty sexy. They have a nice Asian menu.
Bodega in Georgetown has some nice two-tops in the back area. Good Spanish tapas (get the chicken croquettes...delicious!).
Not exactly in the cuisines you were looking for but still romantic and low-key without breaking the bank: Bistro D'Oc, Tabard Inn, and Zola.
Mendocino Grille in Georgetown, seafood is one of its strengths (along with charcuterie), although it is not a seafood restaurant.
Check out the menu. I hope this is not too pretentious for you.
For the most romantic and not overly pretentious (this is DC after all), you have to go to Old Town Alexandria (Virginia) and Restaurant Eve. But don't go for the Tasting Room (expensive), just the bistro.
Everyone's view of romantic is different, but for me I think Cava on Capitol Hill is romantic, I especially like sitting outside.
I also think Palena Cafe although busy is very romantic.
Sonoma to me can be romantic too (and they have the lounge upstairs if you want to hang after dinner). I also think the seafood dishes there are good.
Regent Thai is so pretty inside I think it could be romantic too.