Points and questions for discussion:
1) It is at best impolite to date or sleep with your good friends' exes.
Common wisdom is that in most circumstances, dating your friends' ex can lead only to strain. Your friend will likely feel betrayed or at least uncomfortable. If the break up was acrimonious, you and your friend and your sweetie/her ex are unlikely to be able to socialize together comfortably. If the breakup was friendly, your friend may still feel hurt. And if you keep your relationship secret, you're threatening your friendship.
In rare situations, friends can look at each other and say "It didn't work for the two of us, maybe it'll work for the two of you," and it's nice when this can happen.
So I would submit that as a rule, one should think very very carefully before getting involved with one's friends' exes, and be aware that except in rare circumstances one's friends are unlikely to be o.k. with it.
Once upon a time, I introduced an ex to a friend who would become my housemate. At the time I introduced them, the ex and I were trying to be friends. The friendship went pretty sour and said ex decided I was the bane of all mankind. When my housmate had him over for tea, things got pretty uncomfortable-he would sit in the kitchen and glare at me whenever I passed through. Given that the kitchen was en route from my room to the bathroom, my passing through was inevitable.
2) Assuming open relationships, is it then impolite to date or sleep with your friends' sweeties?
Of course, I have fewer popular resources to guide me on this one, and also less experience. Poly experience the first was with someone whose primary was certainly a member of my community, but not necessarily a close friend. Poly experience the second was weird, because it started out as a fling and both relationships started more or less at the same time. She wasn't a friend, but we had been friendly.
3) If you find yourself getting it on with a friend's ex or current sweetie, should you inform your friend of this? At what point?
I should make it clear that I'm not referring to anyone one knows socially and doesn't dislike as a friend. Definitions of friendship vary and even among the people one calls "friend" there are varying degrees of closeness and intimacy.
I think, though, that most people would agree that they are more loathe to hurt the people they consider friends. I mean, if a friend tells me a story about something bad that happened to a friend of theirs, I feel badly, and I may sympathise, but chances are I don't feel as bad as I would if the same bad thing happened to the person I consider my friend. Chances are, I'm not going to worry about my romantic interests' exes if they're unknown to me.
Certainly, if my current sweetie starts seeing someone, I deserve to know as soon as possible. Just as I should tell my sweeties when I start seeing someone new. If I don't feel I can tell them or they don't feel they can tell me, then we have Serious Relationship Problems that go beyond mere manners. But if I start seeing a friend's sweetie and I'm pretty sure that my friend has been made aware of this, then am I under an obligation to check in with my friend?
4) It is rude to ask someone how they plan to vote in an upcoming election.
sabtabby and I have discussed this at length.
So has her crowd. The Gentleman and I have more or less agreed that it's at least as impertinent a question as asking someone how much they earn or how much their house cost-there are circumstances under which it wouldn't be brazen, but they are specific and mostly reserved for friends.
So what do you think?