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