I've been thinking about this thread since it was initiated almost a week ago because the question is fascinating to me. My conclusion for myself is as follows. I would want to adopt a "girl" role completely and permanently, I'm pretty sure, under the following conditions, but not otherwise:
1. My wife would long to dominate and use me.
2. Although (1) seems downright unfriendly, her whole approach would be loving and in a way "gentle." She wouldn't raise her voice to me. She would simply tell me what to do in a kind, feminine way, and I would do it without hesitation or thought.
3. She would actually enjoy subjecting me to tedium, humiliation, frustration, and a certain amount of pain.
4. She would want to make all the decisions, control all the money, and exercise all the power.
5. She would have very good judgment and her decisions would always be sound and safe.
6. She would give enough attention to our arrangement that I couldn't get away with the slightest deviation. She would be somewhat obsessive about making sure that I obeyed in every detail, and about punishing me if I didn't, even for unintentional mistakes.
7. She would never get tired of the arrangement.
That's about all I would need

It may seem odd that a person would want this, but my current wife and I actually tried this for a few weeks and they were the happiest time of my life. I didn't even begin to get tired of being a slave, but my wife grew weary of her role. I don't blame her and our marriage is still wonderful.
Lisa