They kiss on the Falcon and then right before Han is going to be put in carbonite, but do Han and Leia have sex off screen? Their relationship seems to be too deep to just be on kissing terms. Perhaps they did it in their hotel room on Cloud City.
shareConsidering Leia as princess and general, I wouldn't expect it. Back then Luke and Han were both kind of fighting for her. So I interpret the scene in the carbonite chamber as the point where the true feelings gets revealed. In my head canon, they only got intimate during/after Episode 6.
But of course, who knows, maybe they were totally 60s/70s vibe Friends with benefits. So casual, meaningless sex could be of course possible. But that does not belong into a fairy tale like Star Wars, the honey gets only shared after the final victory and that was in Episode 6.