I think Sally is probably hypersensitive to real and percieved judgment because her experience with her religious/conservative family has not been the most positive.
I think you can both understand her feelings and validate them, but also help her see through it all and give Mary the benefit of the doubt.
It sounds like Mary has been outwardly supportive and even if she is having a bit of an internal wrestling match, it seems by her behaviour that she has chosen to prioritize Sally’s friendship over any engrained moral qualms about her sexual orientation.
If everyone really is as close as it sounds like they are, they should be able to have an open and honest talk about it. Maybe not today at the other wedding, but perhaps they can get together for brunch in the morning or something? And Sally can tell Mary her concerns and give her the chance to either confirm them or put them to bed.
If she does talk to Mary about it though, she needs to make sure she doesn’t come off accusational. Because she really does not have ANY actual reason to believe that Mary disapproves and it’s not fair to Mary to be forced into s defensive position.
Sally should approach it as an issue that she is struggling with and needs Mary’s reassurance on. “Hey, I understand why you felt it best to decline being a bridesmaid in my wedding, but I have felt myself worrying that it’s actually because you’re uncomfortable with me marrying a woman, and I don’t like wondering that kind of thing about you. You’ve always been a good friend and I want to believe that you support me and accept me so I need to put these thoughts to bed.”
ETA: I went back and read your updates more thoroughly and it seems Sally already asked Mary (in a pretty confrontational way) and Mary said it’s not because she’s gay.
I’m sorry, but Sally is behaving really poorly here. Again, I get why she is particularly sensitive, but she is responsible for managing her emotions and it’s on her to believe her friends and take them at face value. If she can’t or won’t do that, then she is going to destroy her friendship with Mary and the fault for that is entirely hers.
Despite Mary being outwardly supportive, Sally accused her of being anti-gay and proceeded to put her on the defensive. I’d be white upset about it if I was Mary and I’d expect an apology.
But instead of apologizing and wanting to smooth things over, Sally is considering just not inviting Mary to her wedding at all… Sally is being a pretty shit friend in all this and honestly, if I were you and/or Beth, I’d be giving her some tough love about it…