And Superboy Makes Three

Captain America (comics) meets the Young Justice team by accident. When he does, he is shocked by Superman's apparent lack of interest in his "son", Superboy. Seeing how he's going to be marrying Iron Man soon anyway, they decide to adopt him. And nearly split the league in two in the process.

Found here: And Superboy Makes Three

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This fanfic contains examples of:

 * Actor Allusion: Tony Stark references how Steve Rogers looks a bit like Johnny Storm. Chris Evans played both Johnny Storm in the Fantastic Four films and Captain America in the Marvel Cinematic Universe.
 * A Dog Named "Dog": Superboy gets a pet dog and goldfish. Three guesses to what he names them. And the first two don't count.
 * He got Fish because he thought it would be a nice gesture towards Kaldur. Kaldur and Fish constantly argue.
 * Tony constantly lampshades Connor's lack of creativity.
 * Made worse (more hilarious) when Superboy declares that he wants a young sister that he will name "Girl" or "Sister".
 * Cast Full of Gay / Everyone Is Bi: Captain America (comics) and Iron Man are a couple, the fic spent ten chapters implying that Kid Flash and Red Arrow are dating, Superboy may or may not have a crush on Robin, and if Robin's comment about Superman sleeping on the couch are any indication Batman and Superman might be a bit more than friends as well.
 * Good Parents: Steve and Tony.
 * Has Two Daddies: Superboy.
 * Human Popsicle: Steve, of course.
 * Oblivious to Love: Steve does not seem to notice that Bruce Wayne is (implied to be) attracted to him.