I seem to be in the minority on number five, but why should we let social mores play such a big role here? The question did not say "without permission." If both of you agree to an open relationship or want to go to a key party, who am I to say that's wrong? You're not hurting anyone by having sex with a person other than your spouse if your spouse is supportive and enthusiastic about it.
Many posters on the Atlas are very traditionalist (as seen in this thread), which I think leads to a lack of understanding here. Marriage suggests a sort of exclusivity sexually, and when such views are so ingrained, it can be difficult to visualize that some couples can be emotionally monogamous but sexually open. Monogamy is deeply ingrained in our culture and there are ever-present messages that adultery is bad, by stigmatizing people who cheat on their spouse and preaching that it is sin from the alter. I think peoples' feelings about this overcomes views toward swinging/open relationships, even if they are in favor of those.
The way people are viewing the question may affect the responses too. I'm pretty sure if you asked "would you be ok having a threesome with your wife and another partner" would garner more responses in favor. Even more if you said the partner was a woman.
Poll
