Maybe it's not about punishment. My theory is that Lois comes across as a normal human woman, albeit very strong and willful compared to average perhaps. Clark is the romantic, who we swoon over partly because of his rare, unshakable monogomy.
In other words, it's obvious that Lois could move on just as well as any other woman. Superman may be a tough act to follow, but Lois can endure. We know she can, so we don't need to "watch over her", so to speak.
Clark, on the other hand, is clearly lost without Lois. So in the mind, his personality and behavior actually *beg* the question "Could he move on? Is he capable of it?"
Thus, that gets explored more often.
My 2 cents.
