The way I thought about it was visually with the circles. SC (suspect class) is the subset and I (immutable trait) as the super set. Given that the plaintiff did not demonstrate that homosexuality is an immutable trait, then you cannot put the member inside the superset and thus it can't be in the subset either. Therefore they fail the necessary condition.
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This helped me a lot.
The way I thought about it was visually with the circles. SC (suspect class) is the subset and I (immutable trait) as the super set. Given that the plaintiff did not demonstrate that homosexuality is an immutable trait, then you cannot put the member inside the superset and thus it can't be in the subset either. Therefore they fail the necessary condition.