This is how it works at my school:
When you're in a Greek system you get a pledge mom (sorority) or pledge dad (fraternity) who looks out for you, gives you gifts, etc. Some people call this their little sister or little brother, but at my school the terms pledge mom and dad are commonly used. Your pledge mom or pledge dad often asks a good friend or sig. other to be their pledge spouse- usually it's another Greek, but it doesn't have to be. So then you have a set of pledge parents who both look out for you and whatever. Usually if you are a pledge spouse and you are Greek, you get your kid gifts with your letters on it, like picture frames or shorts or baseball caps or whatever. Pretty much every house on my campus does this, and I'd never really heard about it being against any rules till I read this site. I guess it sort of makes sense but it's a fun innocent tradition at my school!
Oh, also it's usually not a good idea to ask a boyfriend or girlfriend to be your pledge husband or wife because if you break up, your kid has a dysfunctional pledge family!

A lot of people still have their S.O.s do the job though. My pledge dad is my mom's longtime bf. Of course, sometimes it doesn't matter. My friend has a pledge kid with her ex-boyfriend and even though they're broken up, she talks to her pledge son all the time.