Agnosticism IS the most dishonest position

[The Grand Nudger]

Correct, I'm telling you that in a harm based moral system - if an open marriage is not harmful - then it's not wrong.

Okay. And that's a good indication that harm based moral systems don't work.

Why?  Why would it indicate a problem in a harm based system..if, when harm is not present, a thing is not contended to be wrong?  

There may be other morally relevant facts.  Maybe we can take harm away and an open marriage would still be wrong.  It's up to you to suggest what those other relevant facts might be, if you wish to contend....as you do...that infidelity is wrong as a matter of fact.

I already explained why open marriage cannot be right, it betrays commitment. 

Except when it doesn't.  When an open relationship is more committed than it's contemporary "traditional relationships" - then why would it be wrong?

-and yet people in open relationships can be as and even more committed to their spouses than people in a "traditional marriage". 

Aw, that's touching. I don't know if you really think this stuff through. The thing is, a caring husband would prefer commiting to his one and only spouse, rather than pick up these values from a (bitch) third party being around.

Yes, think this stuff through.  If and when a caring husband does prefer more than one partner...-and- that partner is okay with that - then what would be wrong™ about their relationship?

Why would I?  On account of the simple fact that those peoples very existence is deleterious to your objections?   Tough luck.  They exist.  See above.

I am aware they exist, pal. The thing is, these people are engaged in an immoral, invalid engagement, no matter how sweet and fantastic their *relationship* is. If you feel like you need to take into account every existing relationship out there you think is too cute to rule out, you might want to consider purchasing a new, more efficient moral system.

Maybe you need to hear the gentle sex's opinion about bringing in third parties to learn commitment. 

Good, I'm glad that you're aware that they exist.  Now that we've dispensed with that, and you acknowledge their sheer existence...kindly explain to me what...exactly...is wrong™ about their relationships?

You're struggling. I'm not. In that event, there is nothing objectively..which is to say factually...wrong™ with their relationships. Do you disagree?