The Bible does not provide details as to what sexual practices between a husband and a wife should be considered clean or unclean.
They really should have stopped right there. If GOD isn't worried about what they do in the bedroom, neither should the WTS or the GB. And if there is no guidance, I don't see why a couple is expected not to discuss it with others, if they so please. Even when they agree to stay out of the bedroom, they can't stay out of the bedroom!