Anonymous wrote:What about taking a trip with a close friend of the opposite sex without the approval of your spouse? Would that be considered cheating?
No, not if you didn't do anything. Your point is that it's something you wouldn't do in front of your spouse, ergo it's cheating, which is the standard others have set (which I don't necessarily agree with because it has its limitations, as you are pointing out).
But in case you're being serious and not tongue-in-cheek, the fact that you'd hide a trip with a friend of any kind is a huge red flag in your marriage, even if it is innocent.