If you read the book, it’s made very clear that they’re lesbians.
I think they purposely left the movie ambiguous so that people could interpret it any way they wanted to. I remember when it came out, I was convinced they were just close friends. (Hey, I was very young and very innocent.) My Mom saw it and immediately went with the lesbian interpretation, and I was aghast.
Rewatched it a few years later, and couldn’t believe I’d missed it the first time.
What I’ve always wondered, though, is about the way all the other people in this very small southern town in the “olden days” seemed to just accept Ruth and Idgie’s relationship like it was no big deal. This couldn’t possibly be accurate, could it? That seems like an environment that would be extremely prejudiced against gay people. Maybe the author was just writing the way she wished it was, instead of how it actually was?