They could have just, you know, been adults about it and gradually learned to cope with the situation, or if Superman really wanted to make things easier for her, Clark Kent could have gone to work for The Daily Bugle or something. Instead, he went for the easiest solution, which also conveniently restores things back to how they were before and prevents any awkwardness between him and Lois in the workplace.
In fact, you know what that sounds a lot like? It sounds a lot like Superman had sex with a co-worker, regretted it, and then made her forget all about it so that he could avoid facing the consequences. For those whose lips don't have amnesiac properties, a similar effect can be achieved by slipping a roofie to the other person.
"Some mild heat vision on your hippocampus and you'll feel much better."
In short, the lesson that this classic movie from our childhood teaches us is that there's nothing wrong with having sex with someone who won't remember anything about it afterward.