It may be a byproduct of some dating issues from before her time dating that got passed down to her. Namely, that lots of gay men were in the closet, and ended up leaving their wife/girlfriend when they came out. And that could have carried through to women thinking it was that, because it’s easier to think the person leaving you doesn’t want women, not just doesn’t want you.

Hopefully that mentality just disappears on its own after awhile. I think it’s already on its way out.