It's skipped in the manga, but in the novels, they fucked when they were at Erika's house.
Erika acts very sensually in front of Touka to see what kind of man he is and he doesn't react.
Seras asked him if he's not interested in sex and he explains that his parents often had loud sex in the room next to where he was, like if he didn't exist. That's why Touka associates sex as something disgusting.
Seras asks him if he wants to change this impression with her and he agrees. The next day, Erika told them that they look like their relationship has moved forward.