Those of you who are friendly with your coworkers but no friends, where exactly do you lay the boundary?

I don't do things with them outside of work hours except for doing one dinner out as a group on work trips. Even those that I discuss some personal stuff with (mostly theirs) I keep it to breaks and maybe lunch. It has worked out really well at my current employer for over a decade while also seeing a few of them who do stuff outside of work have very dramatic conflicts and some people left due to that outside stuff.

This followed a few jobs where I had both positive and negative experiences with hanging out with coworkers outside work. Eventually decided that I'm not social enough to need additional friends outside work. Might give it another shot at some point, but don't want to find out something that would sour a good working relationship like the person being a racist who just hides it at work.

I just don't do anything of my personal life with them, it's not hard to draw the boundary.

Don't give my coworkers my number

What do you mean by boundary? You mean the difference between being friends with someone and being friendly with someone? I don't think there is a hard boundary, it's a pretty fluid transition.

they never see my horny side

Or so I hope

We don't hang out or talk outside of work usually.

It's up top the people involved. Yes, there are folks who will try to take advantage of you and coerce you into covering for them. There are also people who actually share interests with you. I've been to a few coworkers homes, and hung out together on off days. There have been other coworkers I wouldn't say 'hi' to on my off days.

This always has worked out pretty naturally for me. I do a lot of work travel and events so you do get close and build relationships with people -- some can even become friends and I'd not be a fan of some way where you never allow yourself to make friends with people you spend 1/3 of 5/7 days with.

Even the people I'm close with and worked for years with I don't call them in a crisis or share super personal stuff -- Maybe the answer is you don't depend on them or have them depend on you for anything.