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.
Social mores exist for a reason. There is risk involved. There may be cases where both partners are genuinely enthusiastic about the other person having sex with someone else, but that doesn't guarantee it won't affect the emotional (including sexual) quality of the relationship, or indeed the emotions of the other people each partner is having sex with.
Poll
