Luthor's marriage proposal to Lois

I have watched all the episodes at least 10 times and I'm still curious why Superman didn't tell Lois about Luthor's evil ways when he was at her apartment. (Lois was asking him if there was any hope for "us".)

Any thoughts appreciated. I have a couple but I want to see what others think about this.

(This probably has come up before but I don't know how I would phrase this to search for it!)

It's always bothered me as well. We have discussed it before and I think most people have said he didn't have any proof and that he was too distraught after being rejection as Clark and then her offering herself to Superman.