Please don't ever make this assumption. I referred an acquaintance from school to my workplace (he was a great project partner, and I was the first of us to get hired after graduation, so why not?), and apparently when he passed through everyone was just... doing work. That was a telltale sign that everyone was grumpy to him.
I had to explain that we basically don't work at all on Friday afternoons because we're too busy drinking, we play pingpong religiously, frequent out of office events on the weekends, and in general it's just an enjoyable crowd. Random conversations happen all the time.
But nope, that one time he came through the office, everyone looked unhappy. Because they were working, I guess.
That sounds ... very strange. Weekends aren't generally speaking "office hours", and more importantly also not "working hours". Having an "out of office event" in my world is when you're doing work, i.e. it's during normal working hours, but you're not at work.
Having frequent work-related "events" on weekends sounds like a total bummer.
This also sounds like a major bummer -- aside from the fact that does smacks (way too heavily) of bro culture, it's incredibly off-putting to anyone who doesn't drink (or who does, but thinks drinking in the office -- or drinking heavily in the afternoon on any kind of a regular basis -- is basically pretty stupid).
But it's okay to tell them what to do?
Um, neither of those are true in any companies I worked at in the US except for a job in high school as a cash register attendee.
Did you suggest that you would feel more comfortable if they sat differently?
This doesn't make any difference for me personally but I understand why some people would prefer that the person not sit like that. However, if you did not express an objection with regards to how that made you feel, I don't think "insisted" is a fair choice of words there.
"He insisted on chewing with his mouth open, I mean, ew. Gross."
The offender is generally expected to read non-verbal, and often non-expressed, communication of the offended's opinion.