i feel like people forget that sometimes characters in fic are written like that because it's a reflection of real life.
people have sex without setting boundaries. people have unprotected sex without talking about their sexual histories or producing recent sti tests. people play with kink without discussing it ahead of time or establishing a safeword. they have anal without 'enough' prep or lube—they may even prefer it like that.
and none of this is really a fantasy. it's all pretty normal. you can feel that it's inappropriately normalised, and you'd probably be right! but it is normalised: one study found that 58% of female undergraduate students on the campus studied had been choked during sex. 20% of those students said that they'd never been asked if it was ok; another 30% said they'd only sometimes been asked if they consented. fully half! (non-paywalled journal article on choking during sex here, including these numbers.) despite a rise in stis of all sorts, condom use is declining. (pdf link to the full text of this study about declining condom use in the us; aidsmap article about an australian study with similar results.)
even when people do talk about things—sex or anything else—they communicate imperfectly. 'yeah, but don't go too far' is consenting and setting a boundary, and also relying that the person you're talking to has the same metric for 'too far' that you do. for some people, 'the trash needs to go out' is a neutral, factual observation; for others, it's a request that the person they're speaking to take out the trash.
even when people understand each other perfectly, people react unpredictably to things sometimes! we behave irrationally! people laugh uncontrollably at funerals, or get angry at the straw that broke their back rather than the enormous load they were already carrying. they get scared and lash out at people trying to help them. when hurt, most people do not instinctively reach for therapy-approved grounding exercises and 'i feel' statements.
pretty much any bad choice that characters could conceivably make is a choice that people make in real life, on purpose, all the time. people do things that can have catastrophic, life-changing effects because it felt like a good idea at the time, or they're leaning into the vibe, or they just didn't think about it all that much, or an infinite number of other reasons.
fiction isn't intended as a guide on the best, safest, and most responsible ways to live your life, and fanfic isn't any different. it's not a narrative flaw to let characters do things that are messy or harmful or downright stupid—it's a reflection of what people are actually like, and not something that authors should feel they have to apologise for.