You tend to find dates and SO's among your social circle. If you're Greek, you're spending a lot of time with other Greeks at unviersity events, social mixers and philanthropy events. It makes sense that over the course of these activities, you might meet someone.
I think sorority women are more likely to date fraternity men b/c of these frequent assocations. But..... Someone who absolutely won't date a non-fraternity member probably does not have great maturity to begin with, so I doubt she's dating much if her criteria is so narrow.
And... a fraternity is a lot more than just parties and girls. If that's all you're interested in, I don't recommend you look into fraternity membership. I don't think you'll enjoy it much.