And the only thing I see is Buffy lying to her best friend.

It wouldn't be the first time though, would it? She lied about being in heaven, she lied about her relationship with Spike.

And if you'd told someone in season 4 that one day Buffy would be sleeping with Spike, they'd react much the same way they'd react if you told them she'd be robbing banks one day. :-)

Characters can do distubing and surprising things sometimes, even if they're being written "in character", whatever that means...