I think you're close to the root of it. The need for a biological legacy is strong no matter any affiliations. Having "non-conforming" children threatens that legacy.
For the groups that inflate this drive, that's all there is. That's the only purpose. No "we as a species".
For the rest of us it might be a little disappointing at the prospect of no grandchildren. But we know that it's ultimately not our decision. And the same outcome could happen regardless of any "conformity".
\m/