I think the point of his confusion is why combine gender minorities and sexual minorities. Part of what I think makes it fit (though this doesn't apply to the inclusion of intersex) is that they tend to be unrecognized minorities that are not united by a belief or central ideology, just that their defining characteristics are, as just mentioned, unrecognized by many. And that is the source of a lot of discrimination. At least, that's how I see the relation is. Intersex, I suppose, it more recently grouped in there because they face similar types of discrimination (at least, I assume so, admittedly I haven't looked much into the topic) and so they join in. There's also, as others mentioned, the fact that grouping all these demographics together gives much more strength to activism by rallying more people.