For me, Virgin is not a gay cruise but it has always felt very open and queer-friendly. I've never felt more comfortable on a cruise than when I first sailed Virgin.
I'd also add, and I don't know if this is directly the Virgin product or who it attracts, but passengers have always been friendly, fun, and outgoing. There are so many spaces to mingle and it's encouraged to strike up conversation with anyone. You don't get that on other lines. Maybe being adult only is a byproduct of that.
These may not be a big draws for everyone, but for me it's what keeps me coming back.