Other than LGBT guilds, I can't think of any other type of gathering focus not directly applying to communication ability (languages spoken), direct play style (pvp, pve, hardcore, raiders, etc) legal considerations (restricting membership of minors) or geographical clustering (combining players of the same timezone).
Personally, I've never considered something like gender identification or sexual orientation when determining my peers for an online game. And honestly, neither topic has ever came up in 10+ years of playing mmos with all sorts of different people from all different walks of life from all over the world. Either people are cool with who you are, or they are probably crappy people.
Is it that these guilds are more respectful to you as a person than guilds who don't identify this way? Or does the topic of orientation or partner preference factor more heavily in to your playstyle or day-to-day play sessions?
As I said before, as someone not in this particular position, I am simply curious and hoping to have a better understanding.
I would say it's because of a few reasons. In the US, LGBT people are being discriminated against, beat down, and otherwise abused by a vocal group of people that feel they have moral superiority over them.
Tie to this the fact that in gaming communities it's very common to use terms like 'gay', 'queer', 'faggot' as pejoratives. If I was gay, I would find these communities offensive, rude, and not worthy of my time.
By carving a server out for themselves, they can try to enforce some level of civility, and protect themselves from this type of asshattery.
Another possible reason, would be for those interesting in meeting people that share their own social background. Whether for friendship or more, it's nice to know that you can ask if the other player is gay or straight without offending or otherwise making someone feel uncomfortable.