That's what irks me. Didn't they ever heard about using a Safe Word?
If u seriously think about it they might not have, they are each other first time and they are both relatively naive ( even the top , he know what he likes but only tried with the bottom ) he might have learned about toys but not the whole bdsm lore
but if he bought the boys there must be an indicative on bdsm,no? it's kind of weird if takamada know he has these urges but never gone around to find out abt bdsm from the internet/when he was buying those toys and not learn about safe words. atleast even without a safe word he can always tell yuu to stop him if it gets too much. i like their relationship but we're 150 chaps in so they should start to develop the plot of what are their do's and no's (unless the author wants to develop this story to a 400 chapter manga )
SPOILER
in the upcoming chapters they talk abt finding a balance point so neither feel bad abt it, still not a safeword in sight but it's progress
Finally
it's not even the 60's anymore bro just learn safe bdsm and rail my dude's brain away without the fear of going overboard (╯°Д °)╯╧╧