Gerald Robotnik was Eggman's grandfather, introduced in Sonic Adventure 2. He made a false deal with Black Doom to create Shadow the Hedgehog in exchange for the Chaos Emeralds, making them both Shadow's dads. They definitely banged and had some kind of toxic mess of a relationship.

Grimer Wormtongue is a nasty little assistant to Ivo Robotnik in Fleetway Sonic the Comic. He is extremely devoted to his cruel master, which is for sure a sexual thing.

Propaganda by @/jorrated [link]

Ebony the Mystic Mog and Pyjamas the Psychic are two characters from Sonic the Comic who appear briefly in a Knuckles story, then become minor recurring characters that take in Super Sonic from the streets. They appear as old friends in their introduction and are implied to live together. If these two aren't married, it's only because Mobius is homophobic.

Mephiles the Dark and Iblis are two halves of the sun/time god Solaris, introduced in Sonic '06. Iblis is kind of just a big fire monster I think but Mephiles orchestrates this whole plot through the course of the game to re-merge with Iblis and form Solaris. Which sounds like some pretty twisted villainous gay devotion to me.