I'm actually, personally, of the thought that Clark should ALWAYS propose before telling her.
I strongly disagree. That's proposing under false pretenses - while she thinks he is something he isn't and doesn't have all the facts and is
unable to make an informed decision. It's dishonest and disrespectful. And it her puts in a horrible position if she says "yes" because then she has agreed to something without knowing what she was really agreeing to. It's basically trapping her, except that she can (thankfully) break off the engagement with the man who tried to lure her into marriage under false pretenses.
This is not the kind of secret to tell anyone who is not fully invested in Clark for the rest of their life. An extremely good friendship (eg Jimmy) or even romantic entaglement, no matter how wonderful, loving, trustworthy ... can end - often badly - and 'tit for tat' hurting is par for the course - not good for Clark and his secret.
Which makes it a really good idea not to trick someone into making a promise under false pretenses - it's apt to make things end a
lot worse than if he'd been honest when things got serious in the relationship and they first started talking about forever. Romantic relationships do end badly sometimes - so do engagements and marriages. By this logic, he should NEVER tell her, even after they are married.
Personally, I have a problem with Clark even having (penis in vagina) sex with any human without telling her the truth, because there is a risk of pregnancy (no matter how slight) and a woman deserves to know if she's running the risk of pregnancy-by-alien. The potential consequences are too huge. Obviously, this problem does not exist in any version where Clark knows he is incapable of reproducing with a human.
