Maybe it's called "Amazing Grace," or maybe it's called a principled commitment, or maybe it's called love. And experiencing it from others (maybe by being adopted by them) inspires it in you.
Then, our harsh mistress would in fact tend to favor its expression and grow its prevalence among the population.
Contrast that with homosexuals, who haven't until recently been having children en masse. But where there is a certain percentage of the population born with a tendency to be gay. So they can always count on having new members.
Unlike being gay, people aren't born with "Amazing Grace" so ... it's a bad strategy that will be weeded out over time. Just like with the Shakers.
This isn't to say that some highly fecund cultural group, like Mormons, can't get away with adopting children here and there. But if they start going too far with it their numbers will start to dwindle.