There is nothing less selfish than havint kids. The amount of sacrifice you have to put in is huge. If you are not on board with that, don't have kids.
You're doing it for a return though: the joy of having kids, planning for the future and so on... I'm sure it's hard work, but it's certainly not an altruistic act.
Even if sacrifice and selflessness were always the same, if one would treat having your own kids differently to adopting a newborn, all other things equal, then at least part of your desire to have kids is some kind of "selfish" motivation, rather than a purely altruistic endeavour to bring up a child or contribute to the workforce of tomorrow (or whatever the ostensible motivation is).
If one actually only cared about performing a service for the child and/or future society why do the exact genes matter?
Unless one actually thinks ones own genes are so superior that they're a service in their own right, in which case, one should be having as many children as possible.
Conversely, if one thinks that by having a child is some kind of cost, either to the child or society (or anything other than oneself), it can be a selfless choice to not have the child, even if you personally wanted one.
Plus, you make strong claims like "nothing less selfish" and yet you have never made sacrifices for anything other than kids... maybe there's something to think about.