Except in those few countries where it is illegal for an unmarried couple to occupy the same hotel room, I don't think that hotels should make any assumptions about the relationship of two guests sharing a room.
If the only thing they know for certain is that one guest is Mr Dave , then address the envelope to Mr Dave.
I see it as similar but no more sinister than someone here on Flyertalk assuming a she is a he - societal norms that when assumed to apply to all and sundry irritate those who don't conform to them and in general provoke feelings of what's the big deal in those who don't ever have to experience what it feels like to be constantly assumed to be someone whom you are not.
I wouldn't personally get too indignant/upset about it and the only way it will change (if ever so slowly) is by people like yourself pointing out to service providers that not every couple is a Mr+Mrs with or without white picket fence.
Expect some flack along the way but I applaud your pointing their incorrect and ingrained assumptions out to them.