I see, where bisexual Buffy comes as a surprise for a lot a lot of people, but I think there is quite some groundwork in TV-canon to make it believable. I also like Joss' acknowledging fluid sexuality. I'm biased here from the way I tick, but to me it feels quite natural that she would feel sexually attracted to one of her girls in this situation. I can also imagine Buffy to be in an emotional place, where she would start an affair, with someone she might not really love back (I fear I'm having a horribly Spuffy biased point of view though):
1) There's the boarding school environment. She even started to fantasize about Xander for Christ's sake. She really really needs some.
2) After her affair with Spike Buffy is sure as hell less easily squicked and more likely to experiment sexually and express uncommon sexual behavior.
3) Buffy was always attracted by beings that match her in power and especially after Spike she also seemed to have accepted that. At least I got the impression that she wasn't sure, if she could go back to sex with standard humans without missing something.
4) From the interview I take it, that Buffy is not really in love with Satsu, so why would she start an affair with her? My very Spuffy interpretation of things ist this: Buffy seems to have given up on love a little. It's possible that she regrets that she took it so slow with Spike in S7 because she might see now that being unsure about her feelings just cost them time that they could have had together. Also if she had convinced him that she loved him back, he might have died a happier man.
Satsu is in love with her. She knows that for a fact and she likes her well enough. So, why not give Satsu what she wants and make her happy? It feels nice to be loved and I don't think Buffy wants to hurt her. She might even have herself convinced that she could grow to love her. Also there is always the "Slayers don't get that old" thing. Does it really make sense to wait for the ultimate feeling or is it better to fulfill somebodies dreams and get some sex and warmth in the process without the risk of major heartbreak.
This is a rather unromantic and dreary point of view but to me the affair makes sense that way.
ETA: I just read #12. Was a good one!