In the comics Clark did not have any of the assurances that LnC Clark should have picked up on. He knew that they had a good relationship, but there had been no obvious declaration of intent from Lois. He could not be sure that it was Clark she truly wanted.
Some people would say you really should not propose until you are sure what the answer will be. I am not sure they are right, but that is one view.
So, while maybe Clark should wait to tell Lois until he feels ready to propose (which I am not agreeing with), he should tell her the secret before he proposes.
