Sleeping with people you know

Last night's episode of Grey's Anatomy got me wondering - when is it a good idea to sleep with someone you consider a friend?

OK, George-Izzy was a Bad Thing. George is married, and both of them were far too drunk to be thinking clearly.

But suppose that George hadn't met Callie. Suppose George and Izzy got a little buzzed, kissed each other... and then had the "you meant it several times" bit. Would that have been a good thing for them? For either of them?

If you get into a romantic and/or sexual relationship with someone early, that can work. But can a friendship turn in to a sexual and/or romantic relationship? Is there some time limit?

Do you ever really get over people? Do you ever know someone you had a sexual relationship with is having one with someone else and not get at all jealous?

You can be a little jealous about someone you're no longer with and still have a good relationship with someone else, but what if you're more than a little jealous?

What is the right thing for Izzy to do? A lot of those looks weren't the looks of someone who had sex with someone and completely regrets it. Part of her, at least, wants George right now. Is it right for her to abandon those feelings?

George is in a different case. We know he feels terrible, of course, but he's married. He took an oath. He either needs to talk it over with Callie, or forget it ever happened.

But what should Izzy do?