You want a red-haired puppy lover guild? Blizzard says "OK!"
You want a hamburger flippers guild? Blizzard says "OK!"
You want a guild built around any inclusive or exclusive group you can dream up? Blizzard says "OK!"
BUT! and this is where the weeping and gnashing of teeth comes from...
You can't advertise it AS a restrictive guild, or a guild built around real life issues in game. Period.
You can use Official World of Warcraft Guild Recruitment forums (paid for by Blizzard, I might add) to advertise your guild, be it GLBT, radical Islamist, drunken bum... whatever. But you can't hop into General chat and say
"GLBT guild recruiting" or
"Christian guild recruiting" or
"Slashdot readers guild recruiting"
If it touches on a sensative real life issue, you don't discuss it public channels in game. Out of game... you go for it. Blizzard will even pay for the bandwidth.
As for marriages? Blizzard has said repeatidly... the tools are there to do it if you want to. Its not officially supported, but its not hindered either. You want two female cows to get married on top of the Twin Collusses? Go for it. Just don't advertise a lesbian wedding in general chat.