Nothing good can come from sleeping with a co-worker. I think the saying, "don't shoot holes in your own boat" is the perfect way of putting this.
No. I have found there is rarely a good result from this. In the one case I've observed, the guy courted the girl and quit before they started a serious relationship. This worked well.
This may be different at larger organizations. I have only worked at small companies where gossip travels fast and doesn't need to travel far.
No, but I have worked with my wife, both as a peer and as her subordinate. No problems there.
Yes, and it turned into a longer term relationship. No negative impact.
I also caught two people in my company having sex in the office toilets. They both got a verbal warning and told to keep it off-site.
Yes, and it was a good life lesson of things not to do
Don't shit where you eat.
No one advises it, for very good reasons, but the truth is that it happens all the time. Co-worker romance is also responsible for a very large percentage of marriages.