I think the metaphor misses something important here, because the number of pebbles seems completely arbitrary. What, if anything, would change if in the pebble-sorters' ancestral environment, preferring 13-pebble heaps was adaptive, but preferring 11-pebble heaps (or spending resources on that that do) was not?

Preferring other people like Larry to be homosexual is adaptive for me. And it is the judgement by others (and the implicit avoidance of that through shame) that we are considering here. That said:

I think the metaphor misses something important here

Absolutely, and the entire line of reasoning relies on conveying the speaker's own morality ("it is second-order 'right' to be homosexual") on others without making it explicit.