My take is that Lois could go back through the portal because it was taking her back to where she belonged, but Clark couldn't go out of his time - he's going to have to live through the next hundred years.
Fortunately, he's able to do that - and he'll still look the same and be ready for Lois when she needs him.
But, OTOH, does something happen to Clark? What is he going to do about WWI and WWII and all the horrors of the Twentieth Century? I can't see him standing by.
