Shoto knows Natsuo is with Shigaraki so he asks Natsuo if he knows why Shiggy was mad at Kaminari.
Natsuo: Yeah, apparently they were married in a video game and the kid stole all their stuff
But Bakugo is being Bakugo in the background on Shoto’s end so what Shoto ACTUALLY hears is:
“Yeah, apparently they were married [*explosions* FUCK YOU, DEKU!] and the kid stole all their stuff.” He just completely misses the “in a video game” part and is shocked with the knowledge that his classmate was apparently married to a villain.
He saves this misheard idea for the next time the villains attack. Shigaraki is busy trying to kill Midoriya (what else is new?) and Shoto walks up behind him and calmly slaps him in the face.
Shigaraki, whirling around: WHAT THE FUCK WAS THAT FOR?!
Shoto: Because you married a child.
The entire fight just stops and everyone is trying to process this but of information.
Shoto: My brother said you married Kaminari.
Shigaraki: married Kamin… HE WAS TALKING ABOUT A VIDEO GAME! DID HE NOT CLARIFY THAT? ALSO HOW DID HE FIND OUT?!
Dabi: I may have told him
Aizawa: This explains so much…
Shigaraki: Do you really think I’m a child predator…? 
Dabi: I mean, you are dating someone a lot younger than you…
Shigaraki: IT’S TWO YEARS, STFU DABI
The pro heroes were called in as backup when the fight started and got there just in time to witness this chaos.
Endeavor: Wait why would my son know about what Shigaraki does anyway?
Shigaraki, trying to come up with an excuse that keeps Natsuo out of trouble: … We’re game husbands in another game
Toga: Game husbands without the game
Shigaraki: SHUT UP, TOGA! WE’RE NOT MARRIED!
Shigaraki: And we never will be-
Kurogiri: Shigaraki Tomura, even All For One himself knows you two are going to get married someday.
Endeavor, dropping to the ground: What has my life come to? Why is my son trying to marry a villain?