The only fraternity and sorority for which this is in writing - aka in the constitutions of the organizations - is Zeta Phi Beta Sorority, Inc. and Phi Beta Sigma Fraternity, Inc. They are bound to each other as brother and sister. (one of the many things I've learned on Greek Chat!

) But that doesn't mean they HAVE to always hang out with each other, date each other etc.
For all the other orgs it will vary from campus to campus. At one school, the DZ's might always hang out with the LXA's...at another school, DZ might always do things with the AXP's. It's really just something that happens, and it doesn't mean you can't date anyone outside of those orgs (i.e., it's not like the T-Birds and Pink Ladies in "Grease.")
Don't pledge a sorority or fraternity because of who they hang out with...because this can change drastically from year to year!!