A few significant developments, and some quasi-incest, are the hallmarks of this classic tale by Siegel and Mooney, in Action 289 (June 1962).
After Linda watches a romantic movie with her parents,she decides that Superman needs a wife, and that neither Lois Lane nor Lana Lang is the right one for her cousin. Her parents warn her not to meddle, but she pays them no mind.
Her first attempt has her try to match up Superman with Helen of Troy. After that fails, she leads Superman into the 30th century, to see the Legion of Super-Heroes when they are grown up. This is the first “Adult Legion” sequence. Cosmic Boy, Sun Boy, Phantom Girl, Brainiac 5, Chameleon Boy and Triplicate Girl are all shown as adults, but the important one, for Supergirl, is Saturn Girl. Sparks do fly between her and Superman when they kiss under the mistletoe.
But Lightning Lad steps in and calls a halt to that. For the first time, we learn that Saturn Girl and Lightning Lad will get married at some point.
Oh, and in a continuity-defying act, Superman and Supergirl invent flight rings for the Legion, to replace the ugly flight belts. The flight rings immediately start being used in stories about the teenaged versions of these characters.
Supergirl confesses her plans to find Superman a girlfriend. Superman explains that he would want someone exactly like her. But they can’t be a couple, because they are cousins. Of course, some cultures allow cousins to marry. But not Kryptonians. Superman has clearly thought a lot about this.
So together they search the universe, and manage to find a super-powered double of Supergirl, Luma Lynai, on the planet Staryl. Superman and this twin of Supergirl hit it off, but she cannot maintain her powers under Earth’s sun, and he will lose his under Staryl’s, so they split.
So Superman and Supergirl decide to move to Kentucky where they marry and settle down.
No, they don’t do that.
But you know they want to.