the offending part is that there should some kind of drive to have your own children or more specifically that the desire to have children is reduced to the need to reproduce.

i have children because i love them and want to care for them. i would adopt children for the same reason. i felt no drive to have children be my own. i could not care less if it is me who reproduced instead of someone else.

of course if humanity as a whole were in danger because there are not enough kids then i would want to contribute that however i can. but even then that could mean helping someone take care of their kids so they can have more kids.