Well, it wasn't their only date, just their only date for a very long time. Clark (as Clark, not as Superman) and Lois dated, and eventually married, in the comics in the 1990s. As Superman, they had had occasional dates throughout the Silver Age and more regular dates in the Bronze Age.
The "and only" date was supposed to be sarcastic, Lynn, implying why Lois preferred Superman to Clark (and basically refused to date Clark again for decades)... because he really was a jerk during the date, in order to keep up appearances of being a fop. After Clark's treatment of her during their date, and then getting kidnapped by those men (other jerks who don't take 'no' for an answer), only to be rescued by a fairly sweet Superman (in comparison), I can see quite plainly why she fell for Superman instead of Clark in the comics. Ironic that the unnamed editor who several pages before told Clark to look into reports of "Superman" tells Lois that her experience was drink induced. If those pages of editor telling Clark to look into Superman had come AFTER Lois told him about her rescue, it would've made more sense. An 1930s editor taking away a good story lead from a female reporter and giving it to a male one would hardly have been surprising. (Sadly, being a 1930s editor is not a requirement of that kind of occurrence to be true for the next 60+ years.)
It is a tad strange how nobody from the orphanage who saw the abandoned baby lift up furniture realized that Superman was the same person all grown up.