Additional context you either didn't pick up on or didn't feel like mentioning: those characters are from DC Comics' Teen Titans team. From left: Kara Zor-El/Supergirl, Tim Drake/Robin #2, Connor Kent/Kon-El/Superboy #2, and Barbara Gordon/Batgirl. So the gag is Robin and Superboy are gay lovers (which I think goes against canon, but I haven't kept up on it for a while; I know Superboy came out as bi recently, but that was the one after this one, who is Superman's son with Lois rather than a clone).