This is the crux of the problem right here: transparency. Mutual honesty upfront is vital. He should have been completely honest with you, which rings alarm bells not just for that reason. I can understand why - in the situation you describe - you would be uncomfortable.

However, I do sometimes think having an opposite sex best friend can work. Obviously, I can only speak from personal experience. My best friend of many years is female. My wife is fully aware, and has no problem with it. Her husband is fully aware and has no problem with it (I am also very good friends with her husband, though not as close). For the record, I've been happily married to my wife for almost 22 years, and the idea of being with anyone else fills me with horror.

At the same time, my best friend is a wonderful blessing to me. She is kind, funny, thoughtful, and we share a lot of mutual interests and views on the world (we also share a very dark and twisted sense of humour). There are times when she leans on me and I lean on her (so to speak) where leaning on my wife would cause extra stress.

Let me give you an example: When you marry, you create a mini economy and become financial dependents. If things are going badly financially for whatever reason (for example job redundancy) that can be stressful. In such a situation, rehashing worries to my wife would simply transfer stress to her and make our lives more miserable. On the other hand, if I talk to my best friend to vent my stress, she is outside the mini-economy of my marriage, can sympathise, perhaps make practical suggestions, then leave the discussion without it having to affect her mini-economy in her own marriage. These kinds of situations are why for me "wife" and "best friend" are best being two separate people. The idea that they must be one and the same is a romantic myth I do not subscribe to.

Bottom line: In some circumstances - and I admit mine are somewhat unusual - an opposite sex best friend can work without any jealousy or problems of any kind. Indeed, my wife credits my best friend with significantly improving our marriage in a number of ways, when she has suggested points of view my wife might have that I had not considered. By the same token, I have done the same for her with her husband on occasion. He is also extremely grateful for my friendship with his wife.

So in essence, whilst I understand the problems in the story you describe, in some cases, the opposite sex best friend thing can work.