I have been to both Gejas and Santorini a couple times each, and I like them both. But I like Santorini better. Gejas is almost a bit too . . . gimmicky? to be romantic. I mean, the food was alright, and truly, it was dim, but I saw someone burned by overheated oil!
If you have big bucks to spend, a totally outrageously expensive place I went to is called Tru, it's downtown, and the hoity toity-est French food you can imagine. Waiters pouring everyone's glasses in unison, escorts to the restroom, the whole nine yards. But in all honesty, I've had better French for 1/4 the price.
I also quite liked Thyme Cafe, food is very good, I guess you could say the atmosphere is romantic.
There is a very nice French cafe called Bistrot Margot, which will be dim in the evening, I think, though less romantic, I guess, than the other places.
My current favorite is a fabulous tapas place called Del Toro in Bucktown. That is some good eating. Try to get a seat in the back, everything is red lighting and red velvet and candlelight. Or if that's too much for you, any of the other dimly lit seats are okay. Good bar, great food, fun and chill place to go.
I know there are more, but I can't think of them all right now. I'm told the restaurant at the top of the John Hancock building is expensive, romantic, touristy, and genuinely very good.
Reservations for all of the above, probably.
I love to eat, in case you couldn't tell.