Kaneki is happily married to Touka. In one omake he realizes with horror that ghoul marriage is initiated by biting someone and leaving bite marks, so that could mean he and Amon are married. In your scenario he likes him as well as Touka. Unlike in the omake where he desperately seeks Amon out to divorce him so he can live with Touka with no regrets he asks him to remain married with him as he has grown to love him as well as Touka. Amon is embarrassed about bite=marriage but agrees

“You mean…you’re married to Amon, too?” Touka points across the room to the former CCG investigator. “The Rabbit is married to the Eyepatch Ghoul who is married to Investigator Amon?”

“Basically,” Kaneki squeaks.

“This is insanity,” Amon mutters. He can’t believe no one at CCG knew this. 

“I concur.” Touka glares at Amon. 

He glares back. “I like you, Rabbit, but I don’t wanna marry you.”

“Same, dude.” Touka sighs.

“I’m okay divorcing you,” Kaneki says to Amon, though he really isn’t. 

Amon flushes. “I mean – you love Touka, right? It’s only right.”

“I do,” Kaneki says, but he hesitates. I want you, too.

“If that isn’t the most pathetic look ever. Both of you.” Touka rolls her eyes. “Ever heard of polyamory?”

“Touka!” Kaneki gasps.

Amon’s face is scarlet.

“I don’t mind. Divorce him if you want, or I’m married to you who’s married to him. It’s fine with me.” Touka sighs. “I just want you to be happy.”

Three months later

“How’s our baby?” Amon bounces into the bedroom where Touka has been napping. “I bought it a crucifix.”

“Huh?” Touka cracks open her eyes. “Oh, that’s actually really sweet.”

Outside the room, Kaneki watches his wife and husband embrace, and wonders how he got so lucky to have them both.