Well, it's against the law for fraternities to be anti-gay... but they often do it anyway without getting caught.
How do you make a gay guy stay away from you? You do things that make him uncomfortable. There are fraternities that will host recruitment events to get many people involved. During those recruitment events, they will observe and analyze those "rushees" and determine who they want and don't want. If they see someone they don't want (like a gay guy), they will simply not focus on encouraging them to attend the next event. If one rushee is gay and acts in a feminine manner, they might do things very manly such as bump each other like football players, do back slaps like tough guys, talk about their wonderful hetrosexual sex lives, etc. Those gay guys might feel discouraged to join and that's what those fraternities want.
What about those who end up in the pledge program and are "outed" later? They simply weed those gay guys out. They haze them, make rules harder for them, set goals that only straight guys would be comfortable with, etc.
What about those who are initiated as brothers and are "outed" later? They simply make those feel left out and move on. I've seen this happen where one brother leaves NTID and goes to a different college... and another brother no longer is involved with their events except the annual banquet.
Yep, it's a sad story. I've got a friend who has helped some of these "outed" people with those situations. 
