>>1039692I see your point but there is some evidence that makes me think Sayaka had bi potential. Let me list them:
1) The whole "I'll marry you" teasing Sayaka did with Madoka. Seems like it wasn't a one time thing too, based on how readily Hitomi was in shipping them and even Madoka's response to it, "Geez Hitomi, you're starting to act like Sayaka!"
2) Sayaka's response (sure it was teasing but still, it came to her mind as a possibility that two girls can have an intense relationship) to Madoka talking about Homura with a KnM kind of reference (see pic).
3) Sayaka's very focused attention on and protectiveness of Madoka. She's the first to notice and comment on Homura's staring at Madoka, pays attention when Madoka wanders off, stands protectively between her and Homura early on, etc. Sayaka distrusts Homura in all the timelines we see, disliking her far more than any of the other girls do, and it almost seems like it could be a bit of jealousy or resentment towards Homura trying to get close to Madoka. Sayaka is certainly the only one to see the level of Homura's obsession with Madoka and well, it's just interesting how she can notice that kind of intense relationship between two girls.
Anyways, that's just my arguments for why I think Sayaka has bi potential.