[adhesive_wombat]

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.

[logical_ferry]

You are intellectualizing too much.

[adhesive_wombat]

I believe that is actually called "thinking", though it can be a shock to some, so take care when starting out.