Straight men don't do so great because while you got the smart, studious breeders, you got the drunks that are always in and out of academic probation. You also got the neurotic, possibly religious types that nearly kill themselves if they get less than a 4.0 or God help them, a 3.9.
Bisexual women don't do so well because every "party girl" is a bisexual. You got a small pool of real bisexuals and then you got the "MySpace bisexuals," girls who are straight but make out with other girls for the purposes of receiving attention. Pretty obvious as to why this group fares poorly.
Gay men probably do the best because their homosexuality excludes them from many other straight men so there are less trappings of doing poorly in school than if they were straight.