Meanwhile, at my job, it's ok that my coworker actually domestically abuses her boyfriend because "that's private" according to my manager. But I get suspended for 3 days and have to take a harassment course because I complained about her behavior.

Please get some serious therapy, and ya should've had the common sense to have covered the wounds which by the way repeated choking will lead to scar tissue building up in neck which will cause serious medical issues like slow blood flow to the brain which will lead to further complications like death.

You should just agree to having domestic violence at home.

Invest in some turtlenecks

I suggest you lay off choking play for a number of reasons. And perhaps you should wear clothes that cover up the marks from your BDSM activities.

I can see HR asking you out if concern but I sincerely doubt this was a group discussion.

At least they care about your well being.

I think you might have overshared a bit in your meeting. I had a meeting once with HR because I took 3 floating vacation days in a row. While ok, it looks odd. When asked why (with a union steward present I add,) my simple answer was "None of your F****** business, can I get back to my job now?" Smart, no, but telling them it's nothing to worry about would be fine.

Tell them, that to supplement their horrible pay, you had to get a job killing people. Then ask when your next pay review is.

They are crazy! What if it really was domestic violence? Having all those people around never helps to confess abuse, if it did the victims would have told all the coworkers already long time ago. They were there all the time. Thats the whole problem that people are shy and don't talk about it.