I think Willow would take Satsu's side on the matter and chastise Buffy for using her to "experiment".
Having first hand knowledge in a similar situation, a good friend of mine pretty much stopped talking to me after my first, um, "experiment". Thinking it was because she was jealous, I also got mad and didn't talk to her either. When we finally did sit down and have a heart to heart, she was upset with me because she felt I had used some girl just to get my kicks. I tried to explain my bisexuality to her and she would have none of it. In her eyes, there was no such thing as a bisexual, just someone who couldn't make up their mind. I've had boyfriends who felt the same way. It's ok to be straight, it's even ok to be gay, but if you're bi, people on the far ends of the spectrums want nothing to do with you.
I've been in a zillion debates over this, and a lot of lesbians that I've talked to have been hurt by women, claiming to be bi-sexual, when in reality, they were bi-curious. And yes, there is a difference. These girls get into relationships with these curious women and end up getting left for a man and they feel rejected and used. So therefore, they automatically judge someone else wrongly that is in that situation.
I didn't mean to go off on an advocacy tangent or anything. I just think that if Willow does get upset, it would most likely be from that point of view. I think she would see Buffy using Satsu and then ultimately hurting her because she's not really gay or bi and Satsu is. I think she would be most upset at the fact that Buffy used this girl, after knowing she was completely in love with her, just to satisfy her curiousity. I mean, obviously, Buffy had no intent of a "real" relationship with Satsu, she pretty much made that clear during the "afterwards" conversation.