My guess is that it's somehow a spandrel of intelligence.
To be immune to selection because it's part of intelligence would imply a strong genetic correlation. Aside from the fact that I am doubtful any such genetic correlation will ever be found (there is no noted phenotypic correlation of homosexuality & intelligence that I've heard of), this still has the issue that homosexuality ought to be decreasing noticeably over time: while intelligence has apparently been neutral or selected for over the past few millennia and so hypothetically could've slowed the selection against homosexuality, intelligence itself...
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