Incest is strictly about blood relations. The step-sibling / step-child porn genre is shorthand for fantasies about getting it on within the cohabitative family or close neighborhood. While there have been some movies that have explored actual incest (porn or otherwise), usually blood incest is regarded as a much more serious matter.

IRL, relationships between steps are verboten not because of the problems with breeding risks but because of how it complicates familial dynamics. It's the same reason fraternization within military units or within work groups is restricted, or at least frowned upon.

Really, we're still expected to ride on a raid to another tribal settlement and kidnap one of their coming-of-age womenfolk the way Hades abducted Persephone¹ Modern alternatives such as singles bars and mixing parties continue to prove wanting.

¹ According to the late Hellenic mythology -- Persephone is queen of the underworld before Hades was ever around, and her abduction is more about pissing off Ceres and justifying why she's sad every six months so we have winter.