Why would an atheist subject themselves to one partner for the rest of their lives?

My partner's been quite nice so far, and I get a shitload of support from them that I'm not sure I could easily live without. So... it's not really always a case of "subjecting".

And what motivates an atheist to bring a new life into this world knowing they will die and become nothing at best and possibly live a miserable life full of immense sadness and pain?

That stuff does play on my mind, but the main reason I think humans should have fewer kids is that we're disastrous for the environment at our current levels of population growth and individual resource use.

But overall, I guess religious believers don't have a monopoly on being motivated by evolved reproductive desire.

they will die and become nothing at best

Yeh but... when you're dead, all the human stuff about "what's it worth" is irrelevant. Like, "worth" is inherently a living-human kind of concept, right? "Worth" and "meaning" don't apply to aggregations of matter-energy like rocks or clouds of hydrogen. It's not that death subtracts away all the worth and meaning you experienced in your life, it's more that worth and meaning are only relevant to lived experience.

It seems like living in technological urban environments is associated with lower rates of human reproduction - EG in some countries populations are shrinking. But I doubt that's because urban people are less godly or because of how they think about the world on an intellectual level.