I think it's harder to be the "sharer" vs the "shared". Like you stated in your well-reasoned question, there's all these positives upfront for the person with the multiple relationships. Being the mono can often feel like more of a compromise.
Personally, I think your question deserves a more emotionally intelligent answer than "I just can't." (As I said, I'm not fond of double standards and that's how this looks.)
Wife has said that she'd expect to be uncomfortable if I ever did want to pursue a relationship but she'd try her hardest to be supportive.