Viktor is a careless adult with absolutely no morals and during a house party everyone is drunk, he lets Yuri come so he doesn't feel left out, and somehow he ends up thinking it's the funniest thing ever to touch the virgin in front of everybody-- he performs all kinds of sex acts on Yuri in front of the other drunk guests to entertain them. Bonus points if he has Yuri top him, extra super bonus points if Yuri is uncomfortable and confused and doesn't want to, and you get my soul if you follow my headcanon and Yuri at ~15 hasn't hit puberty yet (and has the body to show it)