The people who care signed their names[1]. It's not a secret or anything.
Most of the signatories are associated with Australian anti sex trafficking and exploitation groups, although there are several UK signatories and a couple Americans.
A publication[2] by one of the signatories connects the dots. It's driven by the core idea:
"Pornography Use Shapes and Changes Sexual Tastes"[3] which is supported by "In a survey of men involved in online sexual activities, 47% reported being involved in practice or seeing pornography which previously was not interesting to or even disgusted them."[4]
I'm trying to steelman when I say I believe that the authors would agree that this also applies to games with sexual content.
To address your comment specifically, while I see the appeal of consistent moral framework. I personally believe that moral frameworks trade consistency for completeness and rarely accomplish either. You have to assume the value-perspective of the other in order to understand why consistency might take a back seat to some other value we could only speculate on.
1. https://www.collectiveshout.org/open-letter-to-payment-proce...
2. https://www.researchgate.net/publication/391732869_Not_A_Fan...
3. ibid. pg 30
4. ibid
> 47% reported being involved in practice or seeing pornography which previously was not interesting to or even disgusted them
Yeah, right.
It really should be obvious that the natural objection to "if they banned this then why not X" is "they haven't gotten around to it yet" and that the reason they can be more successful is also that they have put their money where their mouth is and also named themselves, something a counter petition will probably struggle with.