I would never allow it. Even allowing for changing emotions on the subject, it just makes me way too uncomfortable... With the cup in, it might be a different story because there's no blood anywhere but in the cup, but otherwise, absolutely a no for me.
If another couple were comfortable with it, it's none of my business, though. As long as it isn't a matter of one person unreasonably demanding it of the other. (I've actually seen blog posts saying that you should dump a guy if he won't give you period oral. That bothered me.)