...She came to my room and asked me what the rest of the conflict as in store for us. It was a touching moment where you can either tell Liara that because you can't be together now, you want to just be friends OR that you'll be together after the Reapers are dealt with in ME3. If you make that commitment to her as I did, commenting that you'll settle down, get married, and have "lots of little blue children", the two of you share an embrace, asses are grabbed, Liara's eyes go black, and it is implied that you have sex.
Afterwards, she leaves the Normandy, thanking you "for the drinks... and the other things."