I am in a relationship with a man who is very good friends with his soon-to-be Ex.

I am 100% confident that they are no longer interested in making their marriage work, yet they have kept a tight bond between them. She wishes us well, and I feel she sincerely likes me. I understand this relationship, as I have had the same with my ex in the past (until his most recent girlfriend put a stop to any contact between us). I do not feel threatened by them in any way, but I do feel that because he will never (and I would never ask him) disconnect in their several weekly and multiple hour visits to her, he puts her in higher regard to me in some ways. The thing I am most having unsettled feelings about is:

He just told me that he uses her as a sounding board for his relationship with me. I was shocked and displeased and told him I didn't want him to discuss ANY of our personal business like that with her. He said he feels there is nothing wrong with it, since (he says) she knows him better than anyone else, and advice from a female friend about his relationship with me is a good thing.

I don't know how to explain to him that this does more harm than good for us. Can you please help give me points to offer on why this is not only disrespectful, but bad for our relationship as a whole?

A triangle always confusing things unfortunately. This is tough as in his eyes he is just talking to her, however that doesn't leave much room for you two to have a relationship. I am not sure if this is helpful, however I think you need to think on this further on how you would like to see the relationship and than take necessary steps.

I understand what you are trying to say, however I know their relationship is over... they have been separated for eight years, and after the first two decided on divorce. They haven't finalized it yet out of simple procrastination, and I made it clear that I would not proceed further into our relationship until it is finalized.

I think that is good that you are saying that to protect yourself. I would just keep my eyes and ears open. I feel sometimes I have not been careful and later found out that I was betrayed in relationships. Just be careful.

Before concurring that this is bad or disrespectful I think a little more information is needed here. Why are they still so friendly? Are there any children involved? If so then that may explain it. I think the best approach for you to take is to emphasize that any problems he feels regarding your relationship should be discussed with you as it is counterproductive to discuss them with anyone else. Asking for another's opinion is at best a temporary fix. If he has a problem with you then you're the one he needs to talk with about it as only you can ultimately make it right. His soon-to-be ex (or anyone else for that matter) can offer him advice and opinions but they cannot fix what he perceives to be your problems ; only you can do that.

They met at 19, had a surprise pregnancy when they were 20, another child after and another surprise pregnancy six years later. So they currently have a 15 year old they are regularly dealing with together. He values her as a person, but they no longer feel compatible as a marriage, and ended it, with the understanding that they would both contribute as much as possible to the raising of the kids.

I am a full understanding of his relationship... my ex and I had the same type of friendship only WHILE WE WERE SINGLE, but that was put to a radical end by his wildly jealous current girlfriend. Now we never speak of anything, not even our children. However, when he was with other women, I was respectful of that and did not carry on to the degree that we did while we were single.

Tell him how it bothers you and if he cares about your feelings he will stop. You are not being unreasonable. Be true to yourself.