I can certainly add a toggle to disable male prey in Endless Mode. In the story, Liz can simply choose not to eat guys. =P
As for the sexual dimorphism angle, here's a fun fact: there are actually species in our universe where only females can swallow other creatures whole! Granted, they're all anglerfish, which are like the example of radical dimorphism, but it does show how evolution can sometimes find paths that might look wacky to an outside observer.
On the Voreside (my name for NPiT's universe), hyperelastic tissues evolved to solve the most pressing problem of hominid reproduction - how do you have both a large head and the relatively narrow pelvis that's required to walk upright? So for hundreds of thousands of years, there's this metabolically expensive tissue that only exists in one sex because it's useless in the other. All the other anatomical changes that the hominid form is simultaneously going through? They're built atop that framework, and suddenly fail (resulting in embryonic death) when a mutation tries to violate it. This is the sort of thing that evolutionary biologists call a "frozen accident" - some structure that's difficult or impossible to evolve around because too much else depends on it.
As for the social angle:
A) Monogamy is far from the human norm - for most of history, successful males sired a disproportionate number of kids, whether through explicit culturally-sanctioned polygyny or by more underhanded means. Anthropology and statistical genetics both bear this out, with some male lineages spreading like wildfire and others dying out completely. You could argue that monogamy was a vital cultural technology which helped stabilize societies beyond the tribal/clan level, but it's not part of our deep evolutionary history.
B) Because one man can impregnate an effectively arbitrary number of women, male intrasexual competition focuses on differential access to women. When it results in violence, as it often does, that violence tends to be secondary to this primary goal - ie, my tribe kills your men in war, but carries away the young women as brides - and is usually pointed away from one's own society. Female intrasexual competition is very different. Since there's no easy way to increase the number of children one woman can carry, her genes are best able to displace others through what's been called "fertility suppression" - minimizing the number of children other women manage to raise. This is usually pointed inward at one's own society, and tends to involve the destruction of someone's reputation, resources, and access to mates. In our universe this is rarely violent, but women on the Voreside have both the biological strength and cultural sanction to adopt a hybrid strategy: a limited amount of inward-facing violence which both achieves fertility suppression of one's rivals, and (when properly safeguarded) also tends to weed out the society's troublemakers.
So the tl;dr version is: the anatomical structures evolved from precursors which were only useful to (and present in) women, and meshed well with strategies for female intrasexual competition.
...also, yes, the logos of the Voreside does fundamentally forbid the concept of male predators. But that one's on me. =P