R vs K selection is a (somewhat oversimplified) model of reproduction. It basically describes two classes of reproductive strategy. R-selected species produce massive numbers of offspring and invest very little into each one (often even banking on a lot of them dying early), and include animals like insects and a number of rodent species.
K-selected species produce few offspring, often one at a time, and invest heavily in the survival and prosperity of each one. A prominent example of a K-selected species would be humans ourselves.
Basically what that anon is saying is that if your waifu was, say, a spidergirl, she likely wouldn't give a shit about any of her children until they'd survived past the point that means they'll likely make it to adulthood, while the human instinct is to care for and protect all of our children.
If I remember correctly, with a number of spider species, the children also eat their siblings, and the mother will often eat the weaker children.
A wife with radically different maternal instincts would probably be hard to deal with.