In my case I didn't got such scene on the first time with Toru. I went all the way with him (ith appened sometime in late september)... And when later started second time, he commented about the failed first attempt, which made me wonder if I had missed something.
Maybe I'm mixing memories, but surely the first time it was at Toru's house, where their parents were gone, they talked about putting the book to practice and so they did... It went to foreplay tutorial, let me remove all his clothes, and then do it with Toru all the way, then slept until morning... But no mother interruption in any moment.