My boyfriend is a Theta Chi. We met freshman year of college before either of us was even thinking about going greek (we had sophomore rush at our school). Neither of us was very sure if we wanted to join, but then we decided to give it a try. I think it was good that we both decided to join our respective org's instead of just one of us because we could understand what the other was going through. It was fun to pledge together because a few of my sisters were dating a few of his brothers so we all were always playing pranks on each other's houses.
I think it is true that if greek people meet their s/o's in college, they are more likely to both be affiliated because greeks tend to stick together. I can't think of very many pairings of affiliated/nonaffiliated people from college, but it is much more common after graduation.
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