The fub has to. Yes. The fub usually is a dilf who breeds, and doesn't just have sex with, but breeds lonely women who are often directionless in life under the nose of their husbands who think everything is all good.
It's fatherless behavior, and it's gluttonous especially if the woman they chase is usually a milf.
Basically for nyaru to be a proper fub she'd have to breed someone who's in a stable relationship, but unsure what to do in life.