Wat? You just stated that it's inappropriate to smile at a co-worker of the opposite sex.

That's sufficiently bizarre that I assume you misread the original post... (I'm applying Occam's razor).

So I need to treat coworkers of the opposite sex and not smile at them? I can only smile to workers of the same sex? Isn't that a form of harassment? Can't I just treat all coworkers the same regardless of gender, sexual identity, race, etc?

Likewise, if the on-call engineer is female, I'm going to text or even call her late at night if I need her help.

I agree about messaging someone of any gender about work at night (without prior agreement), but why would it be wrong to smile at a coworker no matter who they are?

I disagree about messaging coworkers at night. I do it occasionally. Sometimes it's about something humorous, instructional or enlightening that I stumbled on while at home. Other times, it's serious and about work. I've had very productive and candid discussions with my CEO at 11pm. If I have something useful to share, I want to do it while I remember about it, instead of trying to remember the next day.

Thing is, when I randomly message someone at night, I don't expect a response, nor do I view my recipient unfavorably if they don't respond until morning or at all. Of course, it's different if the entire team is working late, but that's only happened a few times in the past 2 years I've been at my current company.

I'm going to carry on smiling at people, irrespective of gender. Sorry.

WTF? You can't smile at a coworker of the opposite sex? Since when? They smile at me often enough. It's a smile, which is a form of acknowledgment.

So many different rules to behave. I just gave up and react to everything the way I am. If I’m not good enough, then at least it’s honest and as is.

I don't think there's anything wrong about instant messaging anyone on a work-approved platform, about work, at any time of day. Expecting a real-time response is unrealistic though.