Now, the thing about Buffy is that she tends to get awfully emotionally involved with anyone she has sex with. First there's Angel, but as her first love and first shag we can forgive her that. Then there's dickwad at college who exists for her to learn that men only want one thing. Then there's Riley who was so jealous and insecure about her power he went off to himself sucked by a vampire. Then there's Spike, which could have been guilt free sex had she not been rather messed up about being dead and alone and was using him.
Throughout all this we can comments about slutty women, from Buffy and the rest of the Scoobies, and we get the show positioning women who enjoy sex as 'bad'. Until maybe season 6 where Buffy realises, with Spike, that sex can be just enjoyable. So, I reckon that we've seen some character progression from her.
Finally in season 8 Buffy matures enough and is confident enough to realise that sex doesn't have to come with emotional baggage, it can just be sex. This is a massive leap forward for her. And she does it with a woman, one of the other slayers, Satsu. Maybe this was just a cheap shot by the writers and artists. Some would say it denigrates bisexual women. I say nay. Buffy aint bisexual she's just having sex. So that aint got anything to do with bisexuality, it's just to do with sex. Now I believe that you can shag someone and not fancy them, and still get something out of it. Which is what I reckon Buffy was doing.
Later, we see Kennedy giving Satsu a pep talk about how this doesn't mean Buffy's been turned:
Now, ignore the awful art and how that looks nothing like Kennedy. It's a little odd how the above scenes seem to infer that Kennedy and Satsu are the only lesbians in the slayer group - I don't know, maybe the slayer potential gene only likes hetero folk..
So. An interesting way to approach this topic. Which isn't really an end to a post but I seem to have run out of steam.