Actually, Angela is bisexual. She doesn't 'turn' any way. And while I wish the writers had been less typical in their choice of Angela to be bi, there's nothing cliched or hackneyed about the actual circumstance of a woman (or man) dating the opposite sex for long periods and then having a same-sex`relationship. That's just life.It'll make a change to see a supporting co-star's character turning gay after being with men for the previous 3 seasons. It's soooooooo original! I think it's called "the Willow effect".
As for Willow, Joss had always intended one of the core 3 Scoobies to be gay. He couldn't decide between Xander and Willow. True to form, I wish he's chosen Xander!