Ok, an ELI5 for you since you seem to need it:

Human beings on aggregate are dumb monkeys. When human see media depicting dumb monkeys doing something, many humans are inclined to try doing same thing.

Slightly less ELI5 -- groups that advocate for things like representation of minorities in media, typically argue that negative representations of ethnic minorities results in increased racism in the general public towards those minorities, as well as the internalisation of those negative stereotypes by minorities. If, for example, black people are only ever shown in media as being gangsters and drug dealers, more black people will lean into that sort of marginalized/negative 'professions', and non-black people will have an inherent bias against them due to the prevalence of the stereotype. If they're shown images/media that shows black people in positions like doctors, lawyers, politicians, etc, then more black people will aspire to emulate those images / move into those fields, and other people will be more accepting/less prejudice against them in those positions. Similarly, if men are shown a ton of step-incest porn, or porn that depicts violence towards partners, they're more inclined to do those things. Step-incest is generally a huge negative outside of the fantasy realm, as things like a step dad + step daughter is pretty damn close to rape in many situations. Sorta like landlord porn -- these things are not ok, but they get normalised through porn and then re-enacted by dumb monkeys like that idiot Ohio mayor or the dude on the cruise line who sexually assaulted and killed his step sister.

The significance of representation in media is basically an established concept. Left-wing sorts want more diverse representation because it leads to more equitable perspectives in the broader community -- Right-leaning sorts often argue against more diverse representation as it chips away at what they consider 'social norms'. Neither side argues that representation doesn't matter, but rather, about who should be represented.

To pretend like this sort of thing doesn't also impact porn, and/or to say that men are just idiots if they emulate things they see in porn, is to go against the idea that media representation matters, and to turn a blind eye to the stats that show these things DO impact dumb monkey behaviours. And to ignore this aspect of media, just in the case of men's sexual urges/porn, is to do so in a way that disadvantages just men, placing all accountability onto the individual man, absolving the system/norms that have facilitated those negative activities/actions. When black people in the 80s were depicted as stereotypes, it was part of what's generally termed systemic racism, which fed into a cycle of black people embracing those stereotypical professions. We don't tend to hold individuals solely accountable for issues stemming from systemic wrongs. So why would we put all the blame on men, in cases when they've been shown step-incest porn, for emulating the tropes they've been taught in step-incest porn?

Besides, porn writers have just been lazy as shit in over-using that trope. You've got sites just reshooting the same boring tired-ass scenes with different shitty actors going through the motions, and hitting pretty much all the same poses and dialogue check boxes. Demand better from your pornographers. Hell, if pornographers were to routinely show more realistic pickups / ways to meet women outside of just "I live with you because we're sorta related, oops my dick slipped in!", maybe there wouldn't be so many incel boys who can't figure out how to approach women.