I threw a party ten or so years back with my then-roommate (a gay man). We had videos playing on both televisions in the house - the living room one was running Russ Meyers movies and the television in my bedroom was running "Dark Shadows", both with the sound turned down, since we had party music on.
As the party was winding down, I noticed that several of my roommate's friends (all gay men) had congregated in my room, turned up the sound, and were watching DS. One of the guys is a NYC DJ. He said that all of his NYC friends were watching it on the Sci Fi Channel at that time, and they all found it fabulous. Of course, I had to agree. We ended up hootin' and hollerin' (as we call it here down South) through several episodes. It was great fun.
Dark Shadows has many qualities that I'd think gay people would relate to. The high drama, the Gothic aesthetic, the divas, the attractive young men, and of course the subtext inherent in the plight of Barnabas Collins - it's all been mentioned here. Overall it offers an escape to a strange, exotic world that's really like nothing else - surely that must appeal to lots of gay people.