As someone who has glimpsed through the raws, there's no emotional cheating nor anything serious that happens between them and Yukino. So no kissing, no complete nudity, no sex, etc. But their relationship isn't without physical touch due to the subject matter (BDSM), so throwing that as a warning for some people who might not be very comfortable with that.
So for the wholesome couple enjoyers, you're pretty safe.
Edit: Though, I'll throw this out now as general content warning, there is another pair of characters that's part of the story; you can see the girl on one of the vol covers, and their story is way less wholesome. And may straight up require TWs.