Sam –
Pashut P’shat is that the Beis Yosef held that a man and his wife is always Derech Chibah
That’s pashut pshat?! Come on. Just because it’s more convenient not to have a shverre Beis Yosef doesn’t make it pashut pshat. And that would be shver in any case, because the Beis Yosef himself paskens like the Rambam that between a husband and wife histaklus can be for simple hana’ah and not tayva and therefore mutar when the woman is a Niddah – something he would not say by ordinary arayos – and now you are going to say that pashtus is all chibah of married people is chibas bi’ah, more so than unmarried people?
My point was that implying that stam negiah is a d’oraisa without qualifying, especially in the context of this discussion, is misleading, and I still think so, despite the fact that there are people who are of that opinion.
As for the question itself, certainly in this situation where the negiah is clearly not related to negiah of chibah, it would be permitted beyond any doubt. It is not any different than a doctor.