I'm updating my response. My sympathy towards Clark not telling Lois his secret until after proposing is based on my assumption that he has never told anyone. (Alt-Lana knows, but then Alt-Lana is engaged to Clark, and it is never explicitly stated if she found out before or after the engagement.)

Some stories have a young Lana in on the secret. If one assumes Clark felt the secret could be shared with Lana, then yes, he should have told Lois sooner, IMHO.

"Hold on, my friends, to the Constitution and to the Republic for which it stands. Miracles do not cluster and what has happened once in 6,000 years, may not happen again. Hold on to the Constitution" - Daniel Webster