It really depends on the fraternity you're pledging. Some very dear friends of mine are active in a chapter of an IFC fraternity, and it just so happens that quite a few members in the chapter are gay or bisexual. Coming (or being) out even before pledging is not any big deal for these guys. However, I know this is not the norm.
Above all, I think you have to ask yourself "in what environment will I feel the most comfortable." I think finding a GLO that is best for any one person is a function of two things: (1) determining whether your own goals/ideals match with the GLO's, and (2) whether you can be true to yourself and your friends as a member of your GLO. If you have a great need to be out, right away, and love a particular GLO, but *know* that there's no way you could come out without sever repercussions...well, why would you want to be there?
It may take a little doing to find the best fit for you...and then, it may not be a perfect fit, but then, there's your chance to set a brotherly example and show your GLO's members that having a gay or bisexual brother need not hurt their chapter's image.
Best of luck!
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