Except she told Xander her reasons for thinking the way she did and he had the choice to accept them or not. Also, they were never in a relationship like Willow/Tara and Buffy/Angel. The circumstances aren't comparable, especially since Buffy was, in fact, the leader.

Angel just flat-out didn't think she was capable of making the decision in the first place. That's sexism, no two ways about it. He saw her, pursued her, manipulated his way into her life, slept with her (she was mature enough to make *that* decision, funnily enough), then continually pursued her after that. It's exemplified in one exchange: "Are you still my girl?" Why ask her that if he knew it was doomed? To keep her on a string, of course, not unlike Drusilla. And the smugness in Yoko Factor, the having her followed at the end of the series...