I think the RSVP/Atlantis trips often work better for couples because most things, in general work better for couples. Going single to anything is frequently like being an unwanted third wheel. And if one goes completely alone, not knowing ANYONE on the ship (like I usually do), it can be especially trying. Luckilly for me, I'm pretty outgoing, which helps. Also, Ocean Princess is a "straight" ship, but their Assistant Cruise Director (Ritchie) always tries to organize "Friends-of-Dorothy" Parties, every cruise, and I've heard that several other ships/lines do these too.
Another route is Pied Piper :
which offers escorted gay groups several times a year, especially on the QE2. I found, on a recent one, that it wasn't quite for me - a generally older group (45+).
I've found, on "straight" cruises, I tend to gravitate towards the people working on the ship (pursers/entertainers/staff) who are usually quite willing to strike up a conversation and occasionally invite one to the staff/crew bar downstairs - although I DON'T want to imply anything sexual about these invites.
On another recent cruise (Carnival Victory), I was seated at a dinner table for 8 (5 men, 3 spinsters), and 3 of the men and I got along famously, especially after I casually "came out" to them. Turned out, that one was "unsure" of himself. He's "sure" now - sure family.