I've got to say, whilst I cringed at Buffy's "The hood?" line to Robin, and would have quite happily reached through the screen and punched her when she started talking about how Chloe was weak, and whilst I really liked Jenny Calendar, I don't think Buffy's feelings towards her are motivated by racism. Buffy is hurting, she has lost her lover, and now there is a killer on the loose. To a certain extent, she likely blames herself for it, due to what triggered the change. She is angry, and when she finds out that Jenny is involved, she blames her. And Giles and the rest of the Scoobies side with Buffy because they feel that she needs the support, and probably because they're hurt that Jenny was keeping secrets from them.
I think that this is unfair- Jenny does not know about the effects of the curse until it is far too late to do anything about it. Jenny does not cause Angel's soul to be taken away. I think it is also motivated in part by the fact that Buffy isn't really that close to Jenny- she's a lot closer to Giles and Willow than to Buffy- and whilst she might have been able to forgive Willow, Giles or Xander, it's a lot easier to stay angry at Jenny. I think part of her realises that she is being unfair to Jenny (or, particularly to Giles), which is why she has the conversation with her in Passion- she is still angry with Jenny, but she doesn't want this to affect Jenny and Giles's relationship.
So, yeah, in conclusion, I think in this instance, the writers show pretty clearly that Buffy's feelings towards Jenny are motivated entirely by the situation, not because of Jenny's culture. This is in stark opposition to her comments to Robin, and her mocking of Kendra's accent, both of which are pretty clearly digs at their respective cultural backgrounds and races. Now, whilst I don't think Buffy intends to be racist, or that she hates people of other races, her comments in these instances are stupid, and ignorant, and yes, racist.
Now, having read your arguments, I can kind of appreciate what you mean by how Jenny was treated, but I would argue that this is more an issue with the writers- Buffy is not angry with Jenny because she is a gypsy, but the writers chose to have Jenny be a gypsy who is keeping secrets about her past, they chose to cast her in a more negative light (dream sequence, etc.), and they chose to have her frozen out of the Scoobies. So I can kind of see an argument that the writers were insensitive in their handling of the character, but not that Buffy's treatment of her was motivated by race.
