How come in some schools frats dont let guys into parties?

[pktcougar]

While a lot of it is ratio, most of it is liability. If they aren't brothers or friends of the fraternity you don't know how they'll act. 18-22 y.o. guys are much more likely to break things, start fights or overall damage. An out of control 120 lb girl is a lot easier to manage then a drunk/drugged/beligerent 250 lb guy. This is just from my experience, we were fairly welcoming until issues as above started becoming issues.