Why do most questions on this site talk about a SO and not more specifically boyfriend or girlfriend? What does SO actually refer to?

SO, as far as I understand, means 'significant other', right? But when I think about the term, I don't necessarily think about the partner in a relationship, rather a person that you are very close to, and who matters a lot to you. Yes, that sometimes means boyfriend or girlfriend, but it might as well mean best friend. And actually, when I read the questions on this site, I tend to read SO as best friend, at first, until I realize that the context can't be right for that, so it must mean boyfriend or girlfriend...
Why do most questions on this site talk about a SO and not more specifically boyfriend or girlfriend? What does SO actually refer to?
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