crimethinc said something about this, "we are so obsessed with authority we cant even have sex without thinking about dominance and submission". I think this is an oversimplification and an assumption (that authority is the sole reason people want those things) that ignores all of the complexities of human sexuality.
first of all I want to say, that actually by definition bdsm is practiced between two EQUAL partners. it is just like how if someone slaps their girls butt and says you've been a bad girl during sex knowing that she would be okay with it, that is different from that person slapping their ass as hard as possible as an actual punishment for an action outside of sex/game playing. this is why I say BDSM is actually rooted in equal distribution of power, and respect for each other, consent and boundaries. in fact, some people say it is therapeutic to them because they can reenact traumatic experiences, but be in full control of how far it goes or when it stops.
as for "the line" BDSM by definition, or maybe what is considered good practice, includes a line. novices and abusers may practice without a line. I think the idea of no limits is a turn on to some and thus they use the phrase, and every human has a limit, when you cross the limit intentionally it is no long BDSM but abuse, when you cross the limit due to miscommunication or lack of voice/clarity, you are both bad at BDSM.
does it reinforce power relations or roles? I think not but it can. see above demonstration about the spanking. bdsm is the same thing as liking spanking or your hands held behind ur back, only taken to a physical extreme. the key word there is physical not mental. nothing changes about the actual power dynamics between the people. the way that it can do this is when abusive people are involved in it. for example, a person I recently had a relationship with was previously in a daddy dom little girl role playing relationship. she said that he would hit her too hard despite her expression of this. they had a game common to DDLG where he would add a hit to her spanking if she did something to be punished, like in general walking around doing whatever. games like this that extend to a 24/7 role playing game aren't uncommon or inherently hierarchical. to elaborate, picture two people playing this game and they do it for things that are basically a joke, they hit the person only as hard as they want to be hit, they can opt out, and maybe they discuss what they can and cant assign punishment for, basically just generally respecting autonomy and treating it as a game. now picture two people playing this game and the dom slowly escalates it to where it is a literal way to control and manipulate the other person. then yes there is a problem. but to answer your question as stated, I do not think bdsm reinforces negative patterns in relationships because it isn't causing people to abuse others, savvy abusers just happen to be drawn to it.
the only other issue I can think of is contracts. as someone else mentioned there are sometimes long detailed signed contracts, especially for the 24/7 type things. this makes me have a negative knee jerk reaction for obvious reasons. I watched a youtube video on whether the contract in 50 shades of gray were enforceable. much of it wasn't but some of it was which is why in todays world I would stay the hell away, but if there was no government to enforce contracts it would just be a game prop. this is not the same as signing yourself into slavery, because it isn't real (unless the person is an abuser then ur in some shit)
