This was the season where all the characters got stripped of everything, some literally...
I think Buffy would have slept with Spike eventually, but that Giles' leaving created a void that Buffy wasn't prepared to deal with. She had no idea how to pay bills, keep Dawn happy, direct the Scoobies, plus deal with her own frustrations, and ALSO train and slay. And Giles' leaving just compounded all her fears about being alone. Buffy doubted herself and thus hit the downward spiral a little harder, a little quicker.
I think his leaving upset a lot of little things, under the surface, which helped a lot of slightly bigger things set into motion, but most of those things were already poised to happen and just reacted to a domino effect. Buffy was relying on him to take care of big things, and take care of providing the support (literally and figuratively), and relying on him to always be there. To some degree, the other characters took their cues from her because he wasn't there. But they faltered in their own way. Things got messed up, but I don't blame Giles so much as I just think it was a serious session of "hello, welcome to life on your own, it's nice, but sometimes it can really suck" that everyone was having now that they were all basically becoming adults.
Season 6 is a character catharsis. Which is usually not about the fun so much as it is about the turmoil, and the learning, and the becoming a better person for it.