>>718269649
They teased it in Vol. 18 and it is heavily implied in Vol. 22.
From Vol. 18
>After a long kiss, Louise began to feel in her hips an unpleasant feeling. When she looked to see what it was, she could see that thing. Her face flushed completely, but with a face full of commitment she said:
"...Do we have to do it?"
For a moment she saw Saito put on a pained expression as he bit his lip forcefully. But immediately Saito showed a smile on his face.
"No, that's fine."
"...You do not want to do it? But you are a man and..."
"...And, uh, obviously I want to. But it's too soon, at the moment there are lots of things I should do before... and also..."
"And also?"
"We will do it... once we're married."

And pic related from last volume.