I know many people who are members of those organizations but I never paid attention if any of them married or are in relationships with each other. I guess it is cuz it is not that important to me.
I've seen some Kappa Gamma Men and Phi Kappa Zeta Women unites in a marriage but it does not mean that ALL of them are like that. It is not only exclusive at Gallaudet. Also, You will see at each and every college/universities, there's a lot of sororities and fraternities are mingling/blending with each other. Some of them will marry each other but some of them don't. It varies.
The same idea could be said for Football/Basketball Players hooking up with Cheerleaders or whatsoever. The point is, there will be different kinds of groups attracting the opposite. Some of them would rather stay ahead in the mainstream while some just prefer to stay aloof.