Well, she herself says “下着を脱がなければいいじゃない”. But I guess it was mostly about possibility of her not feeling well? “身体に悪いだろ？一人の体調が悪い時は、しない。”
As a person with periods that usually swept me off my feet during my teen years, before I was legally able to go to the doctor without parents approval, I can certainly relate to that. Especially since it was 10 days of pain/month, not just a day or two.
Even if the girl here seems to not have similar problems, maybe the guy just was overly cautious just in case 
Anyway. I had a serious moral dilemma should I include this week’s sex scene in warnings when I provided them But I figured out, there no trauma, everyone’s happy, and if anyone’s got this far in this book they should be fine with this description 
But yeah, I also have hygiene concerns 
Also… continuing with my rant from last week’s…
So, I like the weirdness of this week’s scene, but again it’s so strict, there’s one ritual you can do when you’re triple, and nothing else. Even kissing has strict rules. 
At least that was my impression.
And… hmm. I can’t stop wondering if マウス has anything to do with Murata’s book titled マウス . Maybe it’s just a coincidence…? 