Can homosexuality be considered a metanarrative, as in the postmodernist term?
I would say that the answer to whether or not homosexuality can be considered a metanarrative is no.
I think it is a narrative in the sense that, like feminism, queer theory attempts to unearth the homosexual experience from its concealment under the traditionally patriarchal bias in history and literature. So, homosexuality is a narrative in that it can be used as a way to provide a, perhaps not comprehensive, but an alternative (from heteronormativity) form of discourse that describes the historical experience and treatment of homosexuality. This is a discourse that is being developed but has historically been omitted. This is, by definition, an addition to the historical discourse on sexuality; not a totalizing metanarrative intended to dominate the discourse. And, more directly speaking, homosexuality itself, being attracted to members of the same sex, is not a metanarrative. It is a biological disposition.
I think that homosexuality, if we think of it in its project of diversifying historical discourse, is not a metanarrative because metanarratives are characterized as being grand, totalizing world views. I would assume that most proponents of equal rights would favor a postmodern world view which is, by most definitions, more diversified and inclusive of difference. Jean-Francois Lyotard argued for multiple, local narratives as opposed to large sweeping metanarratives.
Sometimes, metanarratives are world views developed from theories. For example, one might form a metanarrative from Marxism or Capitalism using either theory to justify the metanarrative. I just don't see how you could build a metanarrative on sexual preference. A stretch would be a metanarrative on the struggle for equal rights, of which homosexuality would play a part. But such a metanarrative would immediately deconstruct itself because its project is diversity; not assimilation.