I think some people are just not meant to have kids, ya know? My poly unit has three people in it, and all three of us agree that we don't want to have kids. Both of the men in our unit has been appropriately "fixed" in harmony with that decision, so we are very sure about it.
And maybe you'll not adopt, but you'll still have opportunity to interact with various kids, and have a positive impact on their lives. Sometimes kids remember the littlest things that we as adults think they would forget.
Sometimes I think, "Uh oh, there won't be any bloodline issuing from Kevin Thompson to demonstrate what his genes' destiny would be." Yes, when I go, the last of my bloodline will go with me. Not so true of my siblings, most of whom have kids. But that quibble doesn't bother me too much. I can't pass on my genes, but I can still pass on my memes. Every person I meet on the street is someone who'll carry away with them some small piece of me. Wherever I go in this world, I influence (no matter how slightly) how that world will be, and how it will become. The butterfly effect.
As others have pointed out, there's lots and lots of kids (and adults) filling this world, and parents are lining up at the door to add even more people. It's actually a good and needed thing if more people would volunteer to not add more kids to that already-overpopulated equation.
I know: the Catholic, Mormon, and more than a few other churches would tan my hide for saying that.
"God wants us to have more kids! What more reason would you need?" Yyyeah. I'd need my own personal conviction, that's what I'd need.
"I am approached with the most opposite opinions and advice, and by men who are equally certain that they represent the divine will. I am sure that either the one or the other is mistaken in the belief, and perhaps in some respects, both. I hope it will not be irreverent of me to say that if it is probable that God would reveal his will to others on a point so connected with my duty, it might be supposed he would reveal it directly to me."
-- Abraham Lincoln