You will love Silversea cruises! I've sailed twice (age: late 40's) and while I was one of the younger passengers, there were other couples below sixty as well. But everyone, no matter what age, seems to bond on these ships. These are well traveled folks and quite sophisticated. You'll find plenty of Europeans, I'm sure, among the passengers.
Gay couples are common today on all ships, especially the luxury vessels. You'll feel right at home...the staff is very warm, not distant, and passengers intermingle a lot.
A fantastic itinerary ... I'd grab it in a second.