37, divorced with no kids, engaged to a wonderful woman who is also divorced with no kids (although she raised 2 from her ex-husband's first marriage). The prospects of children came up before we really became serious--we are both in unanimous agreement that we don't want children to the point it would have been a deal breaker.
As for why--it's almost impossible to put into words. I feel like there's an instinct, a feeling, something that says "this is what I want to do". I've never had that. Neither has she. The thought of children is so foreign to us to the point it's just...awkward for the lack of a better term.
We've been called selfish amongst a handful of other things, told we'd change our minds, that it would change if we did have our own, that I have to keep the family name, etc etc. You name the reason, common to batshit crazy, and we've heard it. Doesn't change a thing.
It's not for monetary reasons, nor health reasons, nor some overarching philosophy. Just that X factor not being there.
And while his has been a part of me my entire life, one of my closest friends just had a scenario where his wife essentially held the marriage for ransom because she wanted a baby. He loves his daughter, of that I have no doubt, but he's completely miserable because the only thing he wanted less than children is losing his first love.