Per quick discussion in comments, here is a rec list of fics where they are a little more feral cat in relationship to each other! I'm doing both Murderbot & Gurathin and Murderbot/Gurathin and I'll give a quick note about each one but like. Read the tags and the summaries.
The Methods Series by Trisshawkeye: https://archiveofourown.org/series/5042446
Gurathin and Murderbot stumble into a non-sexual kink situation, managing to not discuss what they're actually doing right up until Mensah makes them. Rated T.
Pain Tolerance by Gamebird: https://archiveofourown.org/works/73189306
Gurathin thinks he can take the pain of a govmod correction. Murderbot is happy to let him find out. Rated M.
Leave all your love and your longing behind by Aylwyn228: https://archiveofourown.org/works/77906256/chapters/204123836
This is a little different, a longer action-adventure story where things go wrong and Murderbot has to try to keep Gurathin alive. It's a little less intensely about the relationship, but I think that aspect is there enough to put it on this list. And if you like the whump, Aylwyn has plenty more for you. Rated M.
And I long for this mirrored perspective by trinityofone: https://archiveofourown.org/works/67489981
Murderbot is having some of Gurathin's memories, and it is a bit of a dick about it. Rated M.
A Kinder Knife by parthagenon: https://archiveofourown.org/works/68528296
Another take on non-sexual kink, for a Gurathin who has a history of cutting. Rated M. (archive locked)
that never fades by chameleonchanging: https://archiveofourown.org/works/73337496
Soulmark AU but more horror than fluff. I can't really describe this well in one sentence, but it's great. Rated T. (archive locked)
And Other Hidden Places by Sholio: https://archiveofourown.org/works/73779736
Gurathin is hiring people to hurt him. Murderbot finds this objectionable. Rated M.
Improper Use of Data Ports by GroveDaan: https://archiveofourown.org/works/67554296
Murderbot thinks Gurathin's data port is dangerously exposed, and then it starts messing with it. They might not call what happens next sex, but I kinda would. Rated E.
Data Cleanse by all-systems-dead: https://archiveofourown.org/works/72440616/chapters/188633506
Gurathin wants Murderbot to do some maintenance on his ports. They do not discuss boundaries here, and Gurathin worries about it. Rated E.
Privacy Breach by all-systems-dead: https://archiveofourown.org/works/69340741
Murderbot gets curious about something Gurathin is doing with his augments in his alone time. It is very pushy about satisfying that curiosity. Rated E.
reach out from the inside by the_moonmoth: https://archiveofourown.org/works/78659141
Murderbot does not ask for permission. At one point Bharadwaj awkwardly clears the room. I loved it. Rated E. (archive locked)
Why Can't You Love Me Back? by the_moonmoth: https://archiveofourown.org/works/67138186
Gurathin confesses his gross human desires, and Murderbot lets him act on them. They have zero healthy adult conversations about what they are doing; it's great. Rated E. (archive locked)
Halt and Catch Fire by aros_66: https://archiveofourown.org/works/76973681
This is an established relationship story and part of the story includes ground rules, but it might still hit your wanting them to be more feral buttons. Rated E.
Cavity Search by lawfulchaotic: https://archiveofourown.org/works/80304956/chapters/210844486
This is either a fantasy or an AU, and involves Three in the last chapter, so may not be what you're looking for. But it might also be exactly what you're looking for, in terms of wanting no discussion whatsoever and boundaries being not so much ignored as irrelevant. Rated E.
Paradoxical Comfort by periapte: https://archiveofourown.org/works/72501096
This one is a tiny fic out of a much longer (and excellent) series, and I'm including it here for what seems like a moment of unnegotiated discipline (ish) but isn't, quite. I think you might like it, and if you do, you should check out the rest of the story. Rated T.
Finally I'll link one of my own fics, Alien Remnants Made Them Do It: https://archiveofourown.org/works/74819916
I don't know if this will be what you're looking for, because they're trying to do the consenting thing even in a situation where it's not quite possible, which is I think what you are not interested in seeing more of. However, they are jackasses to each other, so there's that. Rated M.
Anyway, I hope you - and anyone else looking for similar levels of (apparent) dysfunction - enjoy some of these!