Most coworkers exist in a different place in your life than your family and friends. They're acquaintances, but not particularly close. After all, the only reason that you and they are together is mere happenstance and not any particular desire to spend time with one another.
Some coworkers, though, don't really understand this and will often transgress the bounds of propriety. Below is a list of things that I think every coworker should understand are just not typically acceptable.
"Can you look at this rash/mark/mole/lesion?"
Unless your colleague is a doctor, don't go to them for medical advice. If it can't be pulled off with a staple remover, odds are that they won't know how to help you. An extreme no-go is anything you have to disrobe to show to somebody. Remember: whatever ugly thing is growing on you will be discussed around the coffee machine for the rest of your career.
"Let me tell you about my children…"
There's a chance that your coworkers want to hear about your children. It's about 0.5%. If they're curious, they'll ask. Otherwise shut your pie-hole.
"I'm leaving early to go to the hospital for an operation."
This only counts if nobody knows why the hell you're going, and you don't tell anybody anything other than this. It's a big gray area: do we ask what you're having done? Is that too intrusive? Does the fact that you told us mean that you want us to ask? Thanks for ruining my whole day worrying about the appropriate response to your potentially grave health problem that might actually be getting botox injected into your ass.
"This girl was so freaky!"
As impressive as your sexual conquests are, remember that you're at work, not in the gym locker room. Keep your speech appropriate for the occasion. By all means, blow off some steam with your male colleagues by regaling them with your freaky girlfriend stories. Just do it outside work hours. And remember that nobody likes a braggart, so even if it's true you might want to soft-peddle it just a tad.
Anything that involves crying
When a woman cries in your office, and nobody died, you really don't have any idea what to do. Run away? Hug her? Kill yourself? Even worse is when a man does. Call me an overmasculine creep if you will, but it's all I can do not to say "man up and stop your bawling, dude."
"I'm having trouble at home…"
Good, keep it there. Unless your homicidal husband is on the way to the office with a gun right now, I don't want to hear about it. My wife bitched me out over the way I do dishes last night, but I'm not dumping that in your lap. This is a double no-no because it always ends in tears.
"You look so sexy. If I wasn't married/dating/working with you/whatever…"
It doesn't matter how true it is, how innocently it's said, what the reason is, or anything else. It's creepy. It's super-creepy if she works for you or is a lower level than you. As Confucius so wisely said, "Never stick your dick in the cash drawer."
"It's that time of the month…"
LALALALALALALALA! I can't heeeeeeeaaaaaarrrr you! Here's a quick tip for women: the only time a guy cares about your monthly cycle is when he wants to have sex with you and this information is pertinent to your availability. Otherwise, we don't wanna know. It's not like I'm jogging across the hall to tell you that my balls itch really bad this morning. Show some consideration, okay? Please? For the love of God, think of the children!