That's true. On the other hand, if the desire to have one's own biological progeny is heritable, then the previous poster's point still stands. Given a mixed population of people who want to reproduce their own genomes and those who don't, over time the balance will shift to favor those who do.

Parents who don't care whether the children are theirs biologically will have a greater tendency to raise children regardless of biological relationship. That means they are more likely to expend their finite resources on other people's children. That increases the likelihood that other lineages will prosper and expand, and decreases the likelihood that their own will.

Therefore we should expect that, with the passage of generations, the desire to have one's own biological progeny will spread and come to dominate the population.

That is, if such inclinations are heritable--or if a heritable desire to have one's own children appears in the population at some point.