Mashuu confirmed he looked at Satoko back then as a mother figure when he was 12, she did that also.
Now he imagined Satoko both as an equal to his age and then to her in seemingly romantic relationship, that's quite a mature way of thinking for a 15yo.
Looks like Motoki is ambiguous, if he was opposed to Mashuu meeting with Satoko, he would straight out deny it but it might get him rebelling and move their relationship forward, instead allowed it and see where it will go.
Guess it's up to Satoko now...