I just came across this thread, and can't resist commenting.
I agree with Ann that there are cultures, including some youth cultures, where women are seen as less valuable than men. One example would be a street gang where the measure of someone's worth is a combination of their physical strength and their willingness to be violent toward others. Men with those values and the women who "buy in" to those values are going to see men as more valuable, because men as a whole are stronger and more agressive than women.
I also agree that Clark is seen as more valuable than Lois, but it has nothing to do with gender. Clark can save the world, and in the LnC universe has already done it. Lois can do great things with her writing, but she cannot save the thousands of lives that Superman can unless she becomes Ultrawoman permanently.
But I would disagree that Lois is expendable. In fact, I would argue that if you read everything in the archive, Lois comes across as a more complete character than Clark. She is smart, beautiful, strong ("Mad Dog Lane") and successful. Lois is the "top banana" that Clark has to catch up to. Perry, one of the two wisest people in the LnC universe, cherishes her. (The other wise person is Martha, who also comes to cherish her.) Yes, she has issues with family and men from the past; if she didn't, she'd either have no flaws or no warmth, she'd be a less interesting character, and she'd either be married already or she would have no interest in a romantic relationship. Female FOLCs identify with Lois and they write her as the character that they would like to be.
Clark needs Lois in a way that she does not initially need him. Clark without Lois is an intriguing contradiction, the strongest man in the world who has trouble surviving on his own. So a deathfic about Clark without Lois is more interesting than a deathfic about Lois without Clark. There are lots of human widows out there and, while I feel sorry for them, I am not particularly interested in reading about them. There are very few Kryptonian widowers out there, which makes reading about them more interesting. Also please note that Lois is irreplaceable. There might be some fics where Clark remarries, but the only popular one that I can think of right now is Nan's "Home" series, and in that one the woman he marries is Lois reincarnated.
Finally, there is one character who is truly expendable, who is routinely killed off in the LnC universe, and that is Jonathan. Clark and Martha mourn him, but that is never the center of a fic. They both manage to move on. It is a literary cliche; the hero must lose his father in order to fully mature. But he retains his connection with his mother, because she is his source of healing and wisdom. While there could be some fics out there where Martha dies and Jonathan is alive, I can't think of any. So it's not an issue of one gender being more expendable than the other.
Just my $.02.
