I've always found naked sex in battle ones pretty stupid. Like how does he get the time to take them off without the girl countering it?
But well if it's sailor moon magic transformation then I can't really argue.
My logical conclusion is that MC is naked 24/7.
アリシア
"ふふっ、驚いた?
凪沙と相談して、今日はちょっと趣向を変えようって話になったの <3"
Arishia
"Fufu, odoroita?
Nagisa to sōdan shite, kyō wa chotto shukō okaeyoutte hanashini natta no <3"
Alicia
"Fufuh, did we surprise you? / Fufuh, suprised?
I had a little chat with Nagisa, and we decided to change things up a bit for today <3" / I talked it over with Nagisa, and today we decided to try out something little different <3"
From latest post.
Speculation;
This seems to be either love or friendship ending type, assuming instead of blackmailing, MC opts out to help them keep the yuri relationship in secrecy in the school those two attend to, and the two choose to reward him by letting him get a piece of the action as a sign of gratitude.