No it's not... I can name you 60 other BDSM subcultures that are more dangerous and extreme than this. For example breath play and choking is legitimately dangerous. It has killed people. Knife play. Gun play. Humiliation, impact play, CNC, toys, etc that Taka uses isn't that dangerous.
And Yuu is okay with it cause 1) that's his boyfriend 2) he enjoys it and gets pleasure from it lol but people have relationships also that are solely bdsm and do even worse than they do. I don't shame. People can live their own lives and do as they do.
Well something being or not being extreme is very relative to ones experience. Like someone who just does vanilla stuff, even anal intercourse might seem extreme :D
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(But yea for Taka and Yu this isn't _that_ extreme. Like sure the long one looks scary and I don't think you can nor should put it in as fast as Taka did, buut it's still thin and Taka seems to know what he is doing, being med student n all. +Taka stopped putting it further in when Yu asked him to.)

JUST TALK ABOUT WHAT YOU WANT, YOUR LIMITS AND BOUNDARIES IN SEX GOOD FUCKIN LORD. EXTREME BDSM ISN'T RARE. AND HE'S NOT EVEN BEING THAT EXTREME. HE JUST HAS A STRONG CONSENT NON CONSENT AND HUMILIATION KINK. LORD.