Personally, what I think Clark would do in this situation is ask Lois to marry him. She would point out that it could be Luthor's child, but he would say it didn't matter. He loves her and wants her to marry him. Once she said yes, then he'd tell her CK=SM.
Clark wouldn't ask this early on. I think it would feel too "shotgun wedding" to him. (Even though he really, really wants to marry her, baby or not.) He would want to make sure that Lois knew he was asking because of his feelings, not because of their situation. If Lois even wants to keep the baby.
