Insemination and fertilization are the driving forces behind male heterosexuality. Sexual attraction is the subconscious desire to reproduce. It's one of the reasons I find bestiality not just disgusting, but also completely lacking in appeal. Even if one could argue the same about incest, at least incest makes sense from the vantage point of human reproduction (and fun fact, at least 10% of all marriages worldwide are incestuous.) Pregnancy creates the ultimate bond. And even though for example, using Mother's Warmth as the basis, Claire and Damon have a substantial bond as mother and son, pregnancy would accentuate it, taking it to its extreme even, amplifying subconscious Oedipal fantasies.
I don't completely buy those who state that they watch this content simply for the sake of sex. Sex is way too important to be treated so casually regardless of pornography's and social media's cavalier representations. It's so important that we can watch 3D animations of fictional likenesses and feel something.