By relying upon all the canon facts you know, have you already imagine what sonic characters can potentially do, have or want on the sly? Even aspects of their personality they don't want to reveal in broad daylight... Because in the end, we surely don't know everything about them.
I appeal to our imagination for the sake of giving rise to entertaining discussions! Let's share our craziest ideas while remaining within the canonical context.
All right, here I go:
On Sonic: As Sonic likes Mongolian throat singing, he may have already tried hard while nobody was here.
On Rouge: She canonically said that she could kiss Shadow. Okaay she likes to flirt with anybody at any time. But the fact she knows it's scarcely possible not only due to his resolute nature, but also by understanding why he restricts himself to not be in a deep intimacy with someone else anymore, can seem like a real challenge she secretly plans to do. Behind the teasing, it might be her guilty desire the prospect of him being thrown off balance by the sexual tension she's able to bring, and notably to break down the corporeal boundary between them JUST for once, in order to get more than an usual "hmph". From her mischievous personality, she may wish to exploit a few of his vulnerabilities. But why especially Shadow? Because he's very steadfast! Sonic can likewise be detached, (he didn't yield to her advances in Sonic Colors, for example), but at least he can be playful in the same way as Rouge, so it won't be totally pleasant. With Knuckles, it's simply too easy. But Shadow? It almost looks unreal. He's the best to not get distracted at any foolishness, and their friendship practically revolves around their partnership, so that's precisely why she can find the challenge pretty tough and exciting: anything that seems bold and dicey are flirtatious to her. That's also why she's a spy who likes stealing, after all.
But she won't precipitate because she respects him, and she certainly afraid of causing a rift in their sincere relationship anyway. (am not a shadouge fan but i can imagine it)
On Shadow: In some way, can we think that all Kill la Kill's fleshly scenes that he had to put up with, subconsciously caused him to have a dirty mind? He may have curiously seen more, and so it can be his unexpected secret.
I mean, he have probably seen THIS scene (⛔ Warning, explicit): https://www.youtube.com/watch?v=z_tEf8l4fvU
On Tails: He recently proved to be a little fresher than we think (ref. "Who's your dad-). I can now imagine his secretive willingness to be more cooler than him, by draw on some villains' quotes.
On Espio: He's a chameleon. He must have suffered some blisters on his tongue.
On Blaze: Since she doesn't like her bust size, does Rouge's on her fantasy file ? Imagine her for one sec in front of a mirror.
En tout cas, feel free to express your opinions, contradict me if you want, I always enjoy having differing points of view anyway and if you ever have shame about your own headcanons, don't: we're here for.