When Committed, What are Healthy Boundaries with Opposite Sex Friends

This is a question for anyone in a committed relationship. What do you think are proper boundaries for your partner to have with an opposite sex friend?
I've had a few discussions about this in another post awhile ago. My situation with my boyfriend has escalated to separation regarding his friendship with another woman. We broke up briefly, then got back together. I am the one that pulls away from the relationship because I feel uncomfortable. My discomfort has been explained to him several times. He asked if we could go to counseling, which we have done, bought some books, and workbooks. When working on a lesson in the workbook he brought up the other woman. I wasn't going to talk about her, but was going to focus on constructive development type dialogue regarding us.
But since he brought her up, I expressed my feelings in an honest, calm manner. He became defensive. After this went on for awhile, he then expressed concern that we should get her opinion. At that point I blew up stating our discussions do not include her, this is OUR relationship. The discussion that was supposed to be constructive became a volcano of words.
Our issue is: last Christmas at a party they were flirting heavily with each other in front of me and my friends. My friends noticed it and brought it to my attention. I had already noticed it and nearly fell off my chair. Prior to that there have been an accumulation of other incidences . . . too much to write about right now . . . After the Christmas party, in January, he asked me about her and I expressed my worries that it looked like they were having and affair. I was very upset. He denied it. He was defensive in denying it. Every discussion we have he is defensive to the point of angry defensivenss stating he doesn't appreciate being falsly accused.
I am not in a positon to know one way or the other. The previous circumstances don't prove they're having an affair. They, potentially, suggest a breach of healty boundaries leading to high probability. I an very upset about this, it is breaking my heart. He responds with anger.
When not discussing this we spend a lot of time together. He tells me he loves me. But he has never apologized. Not for being unfaithful, but just for the misunderstanding, or perhaps, poor judgment and no intentions of upsetting me, and include a statement for this to never happen again. . . . basic things like that.
Because he is defensive in every conversation we have, our conversations never cover any quality content. I feel frustrated, unheard, and unresolved through the course of the conversation My frustration excalates his defensiveness, and we both end up angry. He says my thoughts are wrong and "ludicrous." I feel my thoughts are sincere and very painful.
I'm Ok with being wrong (I want to be wrong). But it seems to me that a sincerely loving guy would be calmly reassuring, apoligize for poor judgement for situations he didn't know would upset me, make some sort of acknowledging statement of our committment, and how to make things right in the future. Is that asking too much?
We did send her a letter very kindly explaining healthy boundaries between she any my boyfriend. But when he sent it to her from his email from him and me, he was hell bent angry.
Anyway, we are on a time out period now because of this. This is the first time in 2+ years of dating we've had such a great challenge. Other than this we have had a wonderful relationship. But this issue is tearing us apart. I feel his defensive anger make him appear guilty. He thinks I have psychological issues. I have expressed to him I've been decieved before and this is a sensitive area for me.
OK - so what do all of you think are healthy boundaries for a comitted person to have with another opposite sex friend?