I was a huge fan of the Anne books as a kid. I read all of them!

I definitely think that Anne's feelings for Diana are more than platonic. The whole "weeping over the thought of her one day marrying" thing is definitely not something I would do for my best platonic friend.

I also find that the romance between Anne and Gilbert, while genuine, is quite sexless. Anne is capable of feeling a crush on a man, she finds Roy attractive when she's in college, but she realises she can't marry him because she doesn't have enough in common with him. She and Gilbert are "best friends" for a long time, and eventually that friend love seems to have deepened. They are partners in life. But the books never depict her swooning after men, the way another character (Ruby Gillis) does. Other girls are attracted to the boys in her class, but instead Anne writes lavishly about how beautiful the different girls are (especially Diana, but also other girls at Redmond when she goes there). Anne is also happy to spend 4 years having a long distance relationship with Gilbert, writing letters to each other (the "gushy" stuff is always omitted). Her love for him seems quite intellectual and emotional rather than physical.

Even when Anne has married Gilbert (and they do have sex, she has many children), she is far more likely to exclaim over the beauty of a woman than she is a man. For example in Anne's House of Dreams she meets a woman with long golden hair and poppies in her belt, and tells Gilbert "that's the most beautiful woman I've seen in my life" or similar.

You could argue "oh LM Montgomery didn't want to allude to men and women having sex, it wasn't the time" but in fact that's not true. There's a book she wrote called The Blue Castle about an emotionally abused woman who enters into a "marriage" with a rakish guy who she's attracted to, because she believes that she's going to die within a year and she wants to experience sex. She and this man initially aren't "in love" and just like each other, so her "marriage" is clearly done because she wants to have sex (and experience the joys of life in general rather than stay stuck with her abusive family), and she finds him attractive. They end up having a wonderful year living in a cabin in the woods, close to nature, and she ultimately finds out she was misdiagnosed and she worries she married her husband under false pretenses as she knew he didn't want to be tied down to someone long term. Shenanigans ensue and ultimately her husband realises he's in love with her and they stay together.

The point is, LM Montgomery was very aware of sex, and very capable of writing a character who was attracted to men rather than women. Another example would be the Emily of New Moon character, who was deeply attracted to "Teddy".

So Anne is unique, I think she definitely is at least bisexual.