My partner and I have done 10 cruises with Holland America and it's not just the alumni price/upgrade offers that keep us going back. There have been a few other gay couples on all our cruises - not large numbers, but mostly people we have enjoyed meeting. HAL is quite good about acknowledging our relationship, and the crew are excellent. We feel very comfortable on board.
We've not yet been to the Baltic, and would like to try out the Prinsendam - so post a report afterwards!